Fix sponsor/backer badge links in README
Fix sponsor/backer badge links in README

I tried clicking on these today and they went nowhere. They need the
`open-collective` prefix to match the current headings.

mikegee/rubocop,NobodysNightmare/rubocop,bquorning/rubocop,haziqhafizuddin/rubocop,jfelchner/rubocop,akihiro17/rubocop,deivid-rodriguez/rubocop,vergenzt/rubocop,sinsoku/rubocop,tejasbubane/rubocop,jmks/rubocop,urbanautomaton/rubocop,scottmatthewman/rubocop,alexcstark/rubocop,haziqhafizuddin/rubocop,tdeo/rubocop,tejasbubane/rubocop,sue445/rubocop,deivid-rodriguez/rubocop,sue445/rubocop,akihiro17/rubocop,meganemura/rubocop,rtlechow/rubocop,jfelchner/rubocop,tdeo/rubocop,palkan/rubocop,petehamilton/rubocop,b-t-g/rubocop,lumeet/rubocop,urbanautomaton/rubocop,bbatsov/rubocop,maxjacobson/rubocop,sue445/rubocop,vergenzt/rubocop,jfelchner/rubocop,smakagon/rubocop,panthomakos/rubocop,vergenzt/rubocop,pocke/rubocop,NobodysNightmare/rubocop,rrosenblum/rubocop,meganemura/rubocop,panthomakos/rubocop,bbatsov/rubocop,maxjacobson/rubocop,petehamilton/rubocop,bquorning/rubocop,deivid-rodriguez/rubocop,pocke/rubocop,maxjacobson/rubocop,lumeet/rubocop,mikegee/rubocop,ilansh/rubocop,alexcstark/rubocop,backus/rubocop,smakagon/rubocop,bquorning/rubocop,scottmatthewman/rubocop,tdeo/rubocop,sinsoku/rubocop,rrosenblum/rubocop,rtlechow/rubocop,petehamilton/rubocop,tejasbubane/rubocop,akihiro17/rubocop,rrosenblum/rubocop,mclark/rubocop,ilansh/rubocop,jmks/rubocop,backus/rubocop,bankair/rubocop,bankair/rubocop,Drenmi/rubocop,iGEL/rubocop,mikegee/rubocop,palkan/rubocop,panthomakos/rubocop,b-t-g/rubocop,mclark/rubocop,palkan/rubocop,iGEL/rubocop,jmks/rubocop,meganemura/rubocop,Drenmi/rubocop