Document the fact that Rouge only recognises lower-case language identifiers

[nisbet-hubbard]

Why:

GitHub accepts both ```Diff and ```diff for the purposes of syntax highlighting, but GitHub Pages will only highlight a ```diff code block.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Documented this caveat.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require a SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing.

[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10
pages/setting-up-a-github-pages-site-with-jekyll/about-github-pages-and-jekyll.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

@nisbet-hubbard Thanks so much for opening a PR! I'll get this triaged for review ✨