Fix broken link to "Grammar and usage" brand guide page

[Hans5958]

Why:

There is a broken link that I found when reading the content style guide. Turns out that some time ago the page name is changed and thus broke the link. I found the correct address of the page nonetheless, and I wanted to fix it.
(Also, I'm not sure which template should I choose, because this is about the repository's documentation, not https://docs.github.com. The change is quite minor so I hope this single PR is okay.)

What's being changed:

I have changed links that points to https://brand.github.com/content/grammar-usage/ so it points to https://brand.github.com/content/grammar.

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging.
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[janiceilene]

Thank you so much for fixing this @Hans5958! I'll update the branch and get this merged as soon as tests are passing 🌟

[janiceilene]

If you haven't already, you can add yourself to the list of contributors by creating a new comment in this PR using these instructions. Thanks again! ✨

[Hans5958]

Hey, thanks for the merge. Glad to help you guys.

@allcontributors add @Hans5958 for bug

EDIT: well...

[allcontributors]

@Hans5958

This project's configuration file has malformed JSON: .all-contributorsrc. Error:: Unexpected token ] in JSON at position 8560