Fix incorrect use of "backslashes"

[atenni]

Regular expressions should be surrounded by forward slashes, but this doc referred to them as both "forward slashes" and "backslashes" in different places.

This PR updates "backslashes" -> "forward slashes".

Why:

Closes https://github.com/github/docs/issues/23123

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

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• [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.

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Content directory changes

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Source	Preview	Production	What Changed
search-github/github-code-search/understanding-github-code-search-syntax.md	fpt ghec	fpt ghec

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

[cmwilson21]

@atenni Thank you for opening a PR and linking it to your issue! ✨

I'll get this triaged for review! :zap:

[atenni]

@nickmccurdy No problem - done :)

My initial instinct is explicit is better than implicit – but given the audience and the fact that there is example code for this on the page I think your suggestion reads better.

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