Fix typo/error in CONTRIBUTING.md (#5559)
Fix typo/error in CONTRIBUTING.md (#5559)

As opposed to most of the possessive forms, `it's` does not mean something that belongs to `it`. `it's` is a contraction of `it is`, and the correct way to write something that belongs to `it` is `its`.
erkyrath/linguist,github/linguist,erkyrath/linguist,github/linguist,github/linguist,github/linguist,christop/linguist,erkyrath/linguist,christop/linguist,erkyrath/linguist