Fix typo/error in CONTRIBUTING.md

[Astroide]

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.

Old sentence :
This often results in someone grasping the opportunity to create a newer and better and more actively maintained grammar, and we'd love to use it and pass on it's functionality to our users.. That meant This often results in someone grasping the opportunity to create a newer and better and more actively maintained grammar, and we'd love to use it and pass on it is functionality to our users., which I think was not the intended meaning.

Therefore, I changed the it's into an its:
This often results in someone grasping the opportunity to create a newer and better and more actively maintained grammar, and we'd love to use it and pass on its functionality to our users.

Checklist:

• I am associating a language with a new file extension.
• I am adding a new language.
• I am fixing a misclassified language
• I am changing the source of a syntax highlighting grammar
• I am updating a grammar submodule
• I am adding new or changing current functionality
• I am changing the color associated with a language

None of the above applies. I am simply fixing an error in CONTRIBUTING.md.

[lildude]

Wow!! It's only taken nearly four years for someone to spot my mistake 🤣

I'll look into the Docker build failure later.