clustering.md: update minor grammar
clustering.md: update minor grammar

hongchaodeng/etcd,rnaveiras/etcd,tguangch/etcd,glevand/coreos--etcd,lumjjb/etcd,gyuho/etcd,rnaveiras/etcd,li-ang/etcd,OyTao/etcd_learning,glevand/coreos--etcd,whuwxl/etcd,a-robinson/etcd,mqliang/etcd,cockroachdb/etcd,dannysauer/etcd,mkumatag/etcd,mqliang/etcd,bdarnell/etcd,dannysauer/etcd,li-ang/etcd,whuwxl/etcd,plasticbox/etcd,endocode/etcd,a-robinson/etcd,mickep76/etcd,tguangch/etcd,fanminshi/etcd,sebrandon1/etcd,bluebreezecf/etcd,rchicoli/etcd,joshix/etcd,ravigadde/etcd,bluebreezecf/etcd,fanminshi/etcd,akauppi/etcd,yudai/etcd,sinsharat/etcd,handsomesun/etcd,etcd-io/etcd,armstrongli/etcd,akauppi/etcd,armstrongli/etcd,etcd-io/etcd,mkumatag/etcd,heyitsanthony/etcd,bdarnell/etcd,handsomesun/etcd,ajityagaty/etcd,YuleiXiao/etcd,plasticbox/etcd,coreos/etcd,jlamillan/etcd,sinsharat/etcd,sebrandon1/etcd,coreos/etcd,endocode/etcd,heyitsanthony/etcd,gyuho/etcd,lumjjb/etcd,lorneli/etcd,ajityagaty/etcd,etcd-io/etcd,OyTao/etcd_learning,cockroachdb/etcd,CrazyJvm/etcd,CrazyJvm/etcd,jlamillan/etcd,yudai/etcd,xiang90/etcd,insequent/etcd,vimalk78/etcd,vimalk78/etcd,ajityagaty/etcd,joshix/etcd,insequent/etcd,xiang90/etcd,mqliang/etcd,mickep76/etcd,rchicoli/etcd,visheshnp/etcd,qqsun8819/etcd,hongchaodeng/etcd,qqsun8819/etcd,visheshnp/etcd,rnaveiras/etcd,YuleiXiao/etcd,lorneli/etcd,mkumatag/etcd,ravigadde/etcd