Update CONTRIBUTING.md with some grammatical changes to improve its readability

The changes are intended to make the document more readable to the user and also to encourage them to become a part of the community through a good experience reading the software documentation. I have inserted a "the" before "users of Software distributed by Gitlab...." and replaced "right, title and interest" with the plural of those words. It is my first MR, so if I made any mistake please feel free to reach out and I will try my best to take care of it. Thank you.