Update writing-mathematical-expressions.md

[ujjwal-shekhar]

Addressed #21569

Why:

Closes #21569

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

I have added the following Note section right at the end of Writing expressions as blocks block.

Check off the following:

• [X] I have reviewed my changes in staging (look for the "Automatically generated comment" and click the links in the "Preview" column to view your latest changes).
• [X] For content changes, I have completed the self-review checklist.

[github-actions[bot]]

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Content directory changes

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Source	Preview	Production	What Changed
get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions.md	fpt ghec ghes	fpt ghec ghes

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

[cmwilson21]

@ujjwal-shekhar Thanks so much for opening a PR! I made a slight edit to your post to link it to the issue.

I'll get this triaged for review :zap:

[dihydroJenoxide]

👋🏻 Hi @ujjwal-shekhar - Thank you for opening this PR 💛 I looked into this and confirmed functionality and it turns out the math fence blocked syntax should function in public repos and now there's a PR open already to fix that bug. Thanks for bringing this to our attention!