Update the placeholder and fallback example in amp-ad.md (#9097)
Update the placeholder and fallback example in amp-ad.md (#9097)


Gregable/amphtml,zhouyx/amphtml,rajkumarsrk/amphtml,fullspeed-sp/amphtml,Mobusi-ads/amphtml,yandex-pcode/amphtml,kamtschatka/amphtml,jdelhommeau/amphtml,koshka/amphtml,smartadserver/amphtml,comScore/amphtml,voyagegroup/amphtml,zhouyx/amphtml,zhucl/amphtml,clawr/amphtml,donttrustthisbot/amphtml,lannka/amphtml,erwinmombay/amphtml,widespace-os/amphtml,camelburrito/amphtml,jdelhommeau/amphtml,jdelhommeau/amphtml,shawnbuso/amphtml,smartadserver/amphtml,dotandads/amphtml,widespace-os/amphtml,ampproject/amphtml,techhtml/amphtml,honeybadgerdontcare/amphtml,rajkumarsrk/amphtml,DarXector/amphtml,spotxchange/amphtml,rsimha-amp/amphtml,pedjoni8/amphtml,jridgewell/amphtml,src-code/amphtml,jmadler/amphtml,zhouyx/amphtml,jridgewell/amphtml,bpaduch/amphtml,honeybadgerdontcare/amphtml,zhangsu/amphtml,media-net/amphtml,luciancrasovan/amphtml,ecoron/amphtml,voyagegroup/amphtml,donttrustthisbot/amphtml,ADITION/amphtml,arealit/amphtml,lannka/amphtml,src-code/amphtml,zhangsu/amphtml,wjfang/amphtml,arealit/amphtml,spotxchange/amphtml,mhc-gh/amphtml,danielrozenberg/amphtml,comScore/amphtml,laterpay/amphtml,shawnbuso/amphtml,danielrozenberg/amphtml,jpettitt/amphtml,alanorozco/amphtml,Gregable/amphtml,mhc-gh/amphtml,fullspeed-sp/amphtml,adiant/amphtml,engtat/amphtml,keithwrightbos/amphtml,ampproject/amphtml,gumgum/amphtml,mhc-gh/amphtml,Mobusi-ads/amphtml,jridgewell/amphtml,cory-work/amphtml,widespace-os/amphtml,prateekbh/amphtml,avimehta/amphtml,tlong2/amphtml,nexxtv/amphtml,vmarteev/amphtml,adiant/amphtml,nexxtv/amphtml,gumgum/amphtml,voyagegroup/amphtml,rsimha-amp/amphtml,dotandads/amphtml,shawnbuso/amphtml,sriramkrish85/amphtml,sharethrough/amphtml,laterpay/amphtml,avimehta/amphtml,koshka/amphtml,cvializ/amphtml,bpaduch/amphtml,danielrozenberg/amphtml,zhucl/amphtml,clawr/amphtml,rajkumarsrk/amphtml,yieldbot/amphtml,bpaduch/amphtml,honeybadgerdontcare/amphtml,gumgum/amphtml,pedjoni8/amphtml,kixer-com/amphtml,bpaduch/amphtml,jmadler/amphtml,sklobovskaya/amphtml,mhc-gh/amphtml,gumgum/amphtml,comScore/amphtml,zhouyx/amphtml,sriramkrish85/amphtml,yandex-pcode/amphtml,lannka/amphtml,donttrustthisbot/amphtml,yieldbot/amphtml,media-net/amphtml,sebastianbenz/amphtml,Gregable/amphtml,yieldmo/amphtml,adup-tech/amphtml,engtat/amphtml,chaveznvg/amphtml,donttrustthisbot/amphtml,adiant/amphtml,jdelhommeau/amphtml,kamtschatka/amphtml,jpettitt/amphtml,zhouyx/amphtml,chaveznvg/amphtml,dotandads/amphtml,spotxchange/amphtml,mhc-gh/amphtml,voyagegroup/amphtml,cvializ/amphtml,pedjoni8/amphtml,ampproject/amphtml,Gregable/amphtml,yieldmo/amphtml,camelburrito/amphtml,yieldmo/amphtml,voyagegroup/amphtml,media-net/amphtml,powdercloud/amphtml,vmarteev/amphtml,widespace-os/amphtml,jpettitt/amphtml,prateekbh/amphtml,lannka/amphtml,tlong2/amphtml,aghassemi/amphtml,prateekbh/amphtml,powdercloud/amphtml,jridgewell/amphtml,cvializ/amphtml,luciancrasovan/amphtml,rajkumarsrk/amphtml,adup-tech/amphtml,kamtschatka/amphtml,wjfang/amphtml,BenDz/amphtml,Mobusi-ads/amphtml,kixer-com/amphtml,comScore/amphtml,zhucl/amphtml,taboola/amphtml,mhc-gh/amphtml,erwinmombay/amphtml,src-code/amphtml,kamtschatka/amphtml,widespace-os/amphtml,ampproject/amphtml,adup-tech/amphtml,rsimha-amp/amphtml,jpettitt/amphtml,engtat/amphtml,avimehta/amphtml,sharethrough/amphtml,ADITION/amphtml,zhucl/amphtml,arealit/amphtml,techhtml/amphtml,avimehta/amphtml,DarXector/amphtml,clawr/amphtml,prateekbh/amphtml,zhangsu/amphtml,luciancrasovan/amphtml,sklobovskaya/amphtml,cvializ/amphtml,kixer-com/amphtml,kamtschatka/amphtml,sklobovskaya/amphtml,spotxchange/amphtml,zhangsu/amphtml,bpaduch/amphtml,sriramkrish85/amphtml,camelburrito/amphtml,src-code/amphtml,DAC-f1h-dev/amphtml,mhc-gh/amphtml,sebastianbenz/amphtml,powdercloud/amphtml,engtat/amphtml,sharethrough/amphtml,danielrozenberg/amphtml,zhucl/amphtml,keithwrightbos/amphtml,src-code/amphtml,alanorozco/amphtml,wjfang/amphtml,techhtml/amphtml,comScore/amphtml,bpaduch/amphtml,sklobovskaya/amphtml,erwinmombay/amphtml,arealit/amphtml,sebastianbenz/amphtml,cory-work/amphtml,clawr/amphtml,tlong2/amphtml,zhangsu/amphtml,avimehta/amphtml,shawnbuso/amphtml,yieldmo/amphtml,donttrustthisbot/amphtml,sebastianbenz/amphtml,wjfang/amphtml,zhangsu/amphtml,Gregable/amphtml,yandex-pcode/amphtml,adiant/amphtml,prateekbh/amphtml,shawnbuso/amphtml,DarXector/amphtml,zhucl/amphtml,engtat/amphtml,jdelhommeau/amphtml,ADITION/amphtml,keithwrightbos/amphtml,yieldmo/amphtml,tlong2/amphtml,smartadserver/amphtml,chaveznvg/amphtml,vmarteev/amphtml,widespace-os/amphtml,Mobusi-ads/amphtml,prateekbh/amphtml,alanorozco/amphtml,aghassemi/amphtml,ecoron/amphtml,smartadserver/amphtml,erwinmombay/amphtml,jmadler/amphtml,kmh287/amphtml,DarXector/amphtml,sklobovskaya/amphtml,kmh287/amphtml,rsimha-amp/amphtml,lannka/amphtml,sriramkrish85/amphtml,nexxtv/amphtml,laterpay/amphtml,techhtml/amphtml,cory-work/amphtml,sriramkrish85/amphtml,media-net/amphtml,keithwrightbos/amphtml,honeybadgerdontcare/amphtml,DarXector/amphtml,koshka/amphtml,smartadserver/amphtml,avimehta/amphtml,luciancrasovan/amphtml,danielrozenberg/amphtml,powdercloud/amphtml,prateekbh/amphtml,fullspeed-sp/amphtml,aghassemi/amphtml,taboola/amphtml,jridgewell/amphtml,kmh287/amphtml,chaveznvg/amphtml,kixer-com/amphtml,sklobovskaya/amphtml,kmh287/amphtml,kixer-com/amphtml,rsimha-amp/amphtml,taboola/amphtml,BenDz/amphtml,ampproject/amphtml,chaveznvg/amphtml,engtat/amphtml,jmadler/amphtml,tlong2/amphtml,adiant/amphtml,yandex-pcode/amphtml,pedjoni8/amphtml,DAC-f1h-dev/amphtml,honeybadgerdontcare/amphtml,koshka/amphtml,yandex-pcode/amphtml,keithwrightbos/amphtml,media-net/amphtml,sharethrough/amphtml,zhucl/amphtml,kmh287/amphtml,voyagegroup/amphtml,arealit/amphtml,ADITION/amphtml,yieldbot/amphtml,camelburrito/amphtml,ampproject/amphtml,media-net/amphtml,engtat/amphtml,wjfang/amphtml,DAC-f1h-dev/amphtml,nexxtv/amphtml,yieldbot/amphtml,DAC-f1h-dev/amphtml,mhc-gh/amphtml,cory-work/amphtml,sharethrough/amphtml,pedjoni8/amphtml,jmadler/amphtml,arealit/amphtml,alanorozco/amphtml,gumgum/amphtml,comScore/amphtml,laterpay/amphtml,vmarteev/amphtml,voyagegroup/amphtml,wjfang/amphtml,zhouyx/amphtml,camelburrito/amphtml,lannka/amphtml,rajkumarsrk/amphtml,alanorozco/amphtml,donttrustthisbot/amphtml,smartadserver/amphtml,powdercloud/amphtml,gumgum/amphtml,Mobusi-ads/amphtml,jridgewell/amphtml,ampproject/amphtml,luciancrasovan/amphtml,jpettitt/amphtml,powdercloud/amphtml,jpettitt/amphtml,spotxchange/amphtml,vmarteev/amphtml,BenDz/amphtml,ampproject/amphtml,koshka/amphtml,media-net/amphtml,jridgewell/amphtml,cory-work/amphtml,techhtml/amphtml,zhouyx/amphtml,aghassemi/amphtml,powdercloud/amphtml,DAC-f1h-dev/amphtml,arealit/amphtml,adup-tech/amphtml,laterpay/amphtml,yieldbot/amphtml,yandex-pcode/amphtml,kamtschatka/amphtml,vmarteev/amphtml,tlong2/amphtml,spotxchange/amphtml,fullspeed-sp/amphtml,danielrozenberg/amphtml,camelburrito/amphtml,BenDz/amphtml,cory-work/amphtml,cvializ/amphtml,jdelhommeau/amphtml,honeybadgerdontcare/amphtml,dotandads/amphtml,taboola/amphtml,taboola/amphtml,alanorozco/amphtml,ecoron/amphtml,Gregable/amphtml,clawr/amphtml,keithwrightbos/amphtml,alanorozco/amphtml,adiant/amphtml,shawnbuso/amphtml,koshka/amphtml,nexxtv/amphtml,vmarteev/amphtml,dotandads/amphtml,nexxtv/amphtml,ecoron/amphtml,adiant/amphtml,keithwrightbos/amphtml,yieldmo/amphtml,chaveznvg/amphtml,yieldbot/amphtml,fullspeed-sp/amphtml,sklobovskaya/amphtml,taboola/amphtml,rajkumarsrk/amphtml,clawr/amphtml,cvializ/amphtml,danielrozenberg/amphtml,ecoron/amphtml,media-net/amphtml,wjfang/amphtml,ADITION/amphtml,techhtml/amphtml,comScore/amphtml,erwinmombay/amphtml,alanorozco/amphtml,sebastianbenz/amphtml,cory-work/amphtml,honeybadgerdontcare/amphtml,rsimha-amp/amphtml,luciancrasovan/amphtml,jpettitt/amphtml,Gregable/amphtml,jmadler/amphtml,camelburrito/amphtml,luciancrasovan/amphtml,adup-tech/amphtml,bpaduch/amphtml,Mobusi-ads/amphtml,rsimha-amp/amphtml,adup-tech/amphtml,jpettitt/amphtml,aghassemi/amphtml,donttrustthisbot/amphtml,pedjoni8/amphtml,BenDz/amphtml,erwinmombay/amphtml,gumgum/amphtml,dotandads/amphtml,BenDz/amphtml,nexxtv/amphtml,yieldmo/amphtml,kmh287/amphtml,kixer-com/amphtml,sriramkrish85/amphtml,yandex-pcode/amphtml,shawnbuso/amphtml,DAC-f1h-dev/amphtml,ecoron/amphtml,danielrozenberg/amphtml,sharethrough/amphtml,src-code/amphtml,rajkumarsrk/amphtml,clawr/amphtml,laterpay/amphtml,taboola/amphtml,sebastianbenz/amphtml,Gregable/amphtml,aghassemi/amphtml,fullspeed-sp/amphtml,lannka/amphtml,DarXector/amphtml,kixer-com/amphtml