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

I hope someone will look further into my editing, in case I missed anything.
FearTheFrail/first-contributions,lauratavares/first-contributions,RickHaan/first-contributions,FearTheFrail/first-contributions,Abhishekchakru/first-contributions,dichaves/first-contributions,Shhzdmrz/first-contributions,RickHaan/first-contributions,dichaves/first-contributions,Abhishekchakru/first-contributions,Shhzdmrz/first-contributions,lauratavares/first-contributions