fix: typo in concurrency.md

[kanmear]

In network request code example ' is typed instead of ;

Fixes - typo

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• This PR doesn't contain automatically generated corrections or text (Grammarly, LLMs, and similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn't use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

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[parlough]

/gcbrun

[parlough]

Nice catch! Thanks for fixing this :)

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