Documentation: fix typo in proxy.md
Documentation: fix typo in proxy.md

This fixes some typos in proxy.md.

Thanks,

dannysauer/etcd,zaphod-concur/etcd,nekto0n/etcd,wantonsolutions/dinv-etcd,ravigadde/etcd,sayden/etcd,whuwxl/etcd,tguangch/etcd,handsomesun/etcd,a-robinson/etcd,ktateish/etcd,insequent/etcd,lorneli/etcd,mqliang/etcd,bluebreezecf/etcd,etcd-io/etcd,jonboulle/etcd,jlamillan/etcd,mkumatag/etcd,mqliang/etcd,ravigadde/etcd,mkumatag/etcd,sofuture/etcd,jonboulle/etcd,mickep76/etcd,ajityagaty/etcd,armstrongli/etcd,wantonsolutions/dinv-etcd,chipironcin/etcd,rnaveiras/etcd,gyuho/etcd,visheshnp/etcd,sinsharat/etcd,swingbach/etcd,tamird/etcd,rnaveiras/etcd,coreos/etcd,fasaxc/etcd,glevand/coreos--etcd,tguangch/etcd,cchamplin/etcd,zaphod-concur/etcd,visheshnp/etcd,bdarnell/etcd,raoofm/etcd,cockroachdb/etcd,westhood/etcd,dannysauer/etcd,akauppi/etcd,rchicoli/etcd,dnaeon/etcd,fanminshi/etcd,OyTao/etcd_learning,qqsun8819/etcd,rchicoli/etcd,achanda/etcd,plasticbox/etcd,glycerine/etcd,glycerine/etcd,vimalk78/etcd,StorReduce/etcd,YuleiXiao/etcd,vimalk78/etcd,coreos/etcd,linuxcer/etcd,achanda/etcd,cchamplin/etcd,HideoYamauchi/etcd,fasaxc/etcd,sinsharat/etcd,mqliang/etcd,raoofm/etcd,yudai/etcd,lorneli/etcd,siddontang/etcd,bluebreezecf/etcd,ajityagaty/etcd,luxas/etcd,linuxcer/etcd,OyTao/etcd_learning,purpleidea/etcd,YuleiXiao/etcd,endocode/etcd,xiang90/etcd,ajityagaty/etcd,Timer/etcd,plasticbox/etcd,westhood/etcd,fanminshi/etcd,shawnps/etcd,armstrongli/etcd,hongchaodeng/etcd,sofuture/etcd,luxas/etcd,joshix/etcd,yudai/etcd,lumjjb/etcd,a-robinson/etcd,qqsun8819/etcd,purpleidea/etcd,whuwxl/etcd,heyitsanthony/etcd,cockroachdb/etcd,fasaxc/etcd,oscarrr110/etcd,etcd-io/etcd,rnaveiras/etcd,Timer/etcd,sebrandon1/etcd,li-ang/etcd,joshix/etcd,ktateish/etcd,insequent/etcd,nekto0n/etcd,xiang90/etcd,etcd-io/etcd,HideoYamauchi/etcd,CrazyJvm/etcd,li-ang/etcd,endocode/etcd,chipironcin/etcd,jlamillan/etcd,handsomesun/etcd,lumjjb/etcd,tamird/etcd,shawnps/etcd,mickep76/etcd,gyuho/etcd,StorReduce/etcd,swingbach/etcd,heyitsanthony/etcd,akauppi/etcd,bdarnell/etcd,wantonsolutions/dinv-etcd,CrazyJvm/etcd,sebrandon1/etcd,siddontang/etcd,hongchaodeng/etcd,oscarrr110/etcd,mkumatag/etcd,sayden/etcd,glevand/coreos--etcd,dnaeon/etcd