Fix grammar error in CONTRIBUTING.md
Fix grammar error in CONTRIBUTING.md

tony-o/rakudo,nbrown/rakudo,rakudo/rakudo,rakudo/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,labster/rakudo,lucasbuchala/rakudo,nbrown/rakudo,tony-o/rakudo,salortiz/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,labster/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,rakudo/rakudo,nbrown/rakudo,Gnouc/rakudo,softmoth/rakudo,rakudo/rakudo,labster/rakudo,nbrown/rakudo,MasterDuke17/rakudo,Gnouc/rakudo,tony-o/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,tony-o/rakudo,softmoth/rakudo,MasterDuke17/rakudo,labster/rakudo,tony-o/rakudo,lucasbuchala/rakudo,tbrowder/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,salortiz/rakudo,softmoth/rakudo,nbrown/rakudo,MasterDuke17/rakudo,salortiz/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,Gnouc/rakudo,tbrowder/rakudo,salortiz/rakudo,softmoth/rakudo,softmoth/rakudo,labster/rakudo,salortiz/rakudo,rakudo/rakudo,labster/rakudo,rakudo/rakudo,tony-o/rakudo,nbrown/rakudo,salortiz/rakudo