Use backticks to format file name in using-submodules-with-github-pages.md

[nguyenalex836]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/pages/getting-started-with-github-pages/using-submodules-with-github-pages

This issue is intended for newcomers to the OS Docs repo! 💛

If you are veteran of the OS Docs repo and are looking for issues to work on, take a look at our help wanted section ✨

What part(s) of the article would you like to see updated?

Use the https:// read-only URL for your submodules, including nested submodules. You can make this change in your .gitmodules file.

Use backticks to format references to the file names per our style guide

Additional information

No response