Add missing scoped_lock to toc.yml

[Rageking8]

scoped_lock is missing from the toc.yml even though the <mutex> header index page has it.

[learn-build-service-prod]

Learn Build status updates of commit 762937d:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/standard-library/toc.yml	✅Succeeded

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[prmerger-automator]

PRMerger Results

Issue	Description
Yaml File(s)	This PR includes changes to .yml file(s) owned by another author.

[v-regandowner]

@TylerMSFT - Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[TylerMSFT]

It looks like someone didn't really finish that topic and didn't come back to adding it. Though it's referenced from the mutex topic so may as well add it. If you get a moment sometime, it may be good to make the topic more 'useful'.

[TylerMSFT]

#sign-off

[prmerger-automator]

Invalid command: '#sign-off'. Only the assigned author of one or more file in this PR can sign off. @

[TylerMSFT]

I own this file. Merging.