Update pong.kv
Update pong.kv

PongBalls' id is not needed at this point of the tutorial; moreover it had a confusing value that was different from the values later on in the text ('pong_ball')
aron-bordin/kivy,vitorio/kivy,Ramalus/kivy,bob-the-hamster/kivy,VinGarcia/kivy,CuriousLearner/kivy,kived/kivy,vipulroxx/kivy,jkankiewicz/kivy,arlowhite/kivy,Farkal/kivy,gonzafirewall/kivy,tony/kivy,kivatu/kivy-bak,manthansharma/kivy,bliz937/kivy,MiyamotoAkira/kivy,hansent/kivy,xpndlabs/kivy,jffernandez/kivy,janssen/kivy,wangjun/kivy,bob-the-hamster/kivy,darkopevec/kivy,LogicalDash/kivy,jehutting/kivy,arcticshores/kivy,dirkjot/kivy,arcticshores/kivy,rafalo1333/kivy,manashmndl/kivy,zennobjects/kivy,darkopevec/kivy,rnixx/kivy,yoelk/kivy,kivy/kivy,wangjun/kivy,arlowhite/kivy,zennobjects/kivy,rafalo1333/kivy,ernstp/kivy,darkopevec/kivy,arcticshores/kivy,jffernandez/kivy,VinGarcia/kivy,Davideddu/kivy-forkedtouch,edubrunaldi/kivy,ehealthafrica-ci/kivy,Cheaterman/kivy,autosportlabs/kivy,niavlys/kivy,xpndlabs/kivy,zennobjects/kivy,matham/kivy,niavlys/kivy,Davideddu/kivy-forkedtouch,LogicalDash/kivy,dirkjot/kivy,matham/kivy,ernstp/kivy,JohnHowland/kivy,bionoid/kivy,edubrunaldi/kivy,cbenhagen/kivy,LogicalDash/kivy,arlowhite/kivy,yoelk/kivy,denys-duchier/kivy,autosportlabs/kivy,MiyamotoAkira/kivy,niavlys/kivy,bliz937/kivy,gonzafirewall/kivy,JohnHowland/kivy,bliz937/kivy,Shyam10/kivy,jegger/kivy,denys-duchier/kivy,kivy/kivy,dirkjot/kivy,Shyam10/kivy,inclement/kivy,manthansharma/kivy,rafalo1333/kivy,niavlys/kivy,janssen/kivy,yoelk/kivy,bionoid/kivy,hansent/kivy,CuriousLearner/kivy,mSenyor/kivy,ehealthafrica-ci/kivy,thezawad/kivy,iamutkarshtiwari/kivy,thezawad/kivy,kivatu/kivy-bak,yoelk/kivy,dirkjot/kivy,cbenhagen/kivy,tony/kivy,youprofit/kivy,aron-bordin/kivy,denys-duchier/kivy,el-ethan/kivy,vitorio/kivy,manthansharma/kivy,manthansharma/kivy,wangjun/kivy,bionoid/kivy,manashmndl/kivy,zennobjects/kivy,iamutkarshtiwari/kivy,manashmndl/kivy,edubrunaldi/kivy,aron-bordin/kivy,andnovar/kivy,VinGarcia/kivy,kivatu/kivy-bak,vipulroxx/kivy,bob-the-hamster/kivy,thezawad/kivy,Cheaterman/kivy,habibmasuro/kivy,bionoid/kivy,KeyWeeUsr/kivy,mSenyor/kivy,adamkh/kivy,autosportlabs/kivy,JohnHowland/kivy,gonzafirewall/kivy,KeyWeeUsr/kivy,CuriousLearner/kivy,janssen/kivy,Davideddu/kivy-forkedtouch,hansent/kivy,ehealthafrica-ci/kivy,xpndlabs/kivy,viralpandey/kivy,angryrancor/kivy,jkankiewicz/kivy,jffernandez/kivy,xiaoyanit/kivy,aron-bordin/kivy,Cheaterman/kivy,JohnHowland/kivy,KeyWeeUsr/kivy,el-ethan/kivy,xiaoyanit/kivy,Cheaterman/kivy,arcticshores/kivy,jkankiewicz/kivy,eHealthAfrica/kivy,Farkal/kivy,Ramalus/kivy,adamkh/kivy,MiyamotoAkira/kivy,kived/kivy,jehutting/kivy,hansent/kivy,jkankiewicz/kivy,viralpandey/kivy,Shyam10/kivy,andnovar/kivy,eHealthAfrica/kivy,wangjun/kivy,youprofit/kivy,Farkal/kivy,mSenyor/kivy,rnixx/kivy,ernstp/kivy,cbenhagen/kivy,vipulroxx/kivy,akshayaurora/kivy,bob-the-hamster/kivy,vitorio/kivy,angryrancor/kivy,viralpandey/kivy,akshayaurora/kivy,youprofit/kivy,gonzafirewall/kivy,matham/kivy,kivatu/kivy-bak,xiaoyanit/kivy,habibmasuro/kivy,habibmasuro/kivy,darkopevec/kivy,eHealthAfrica/kivy,Davideddu/kivy-forkedtouch,janssen/kivy,jehutting/kivy,jegger/kivy,LogicalDash/kivy,jegger/kivy,tony/kivy,inclement/kivy,bhargav2408/kivy,matham/kivy,akshayaurora/kivy,el-ethan/kivy,adamkh/kivy,Shyam10/kivy,jegger/kivy,Ramalus/kivy,KeyWeeUsr/kivy,vipulroxx/kivy,ernstp/kivy,jffernandez/kivy,inclement/kivy,bhargav2408/kivy,ehealthafrica-ci/kivy,adamkh/kivy,Farkal/kivy,MiyamotoAkira/kivy,iamutkarshtiwari/kivy,rnixx/kivy,angryrancor/kivy,denys-duchier/kivy,kivy/kivy,kived/kivy,eHealthAfrica/kivy,angryrancor/kivy,andnovar/kivy,bhargav2408/kivy