Update __m64 reference page

[Rageking8]

Summary:

• Enclose heading in backticks
• Format code snippet
• Fix invalid mention of _m64 (missing an underscore)
• Standardize formatting of __m64 (with backticks and bolded)
• Replace br elements with escapes
• Clean up redundant relative links
• Edit metadata

[prmerger-automator]

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[learn-build-service-prod]

Learn Build status updates of commit fd15660:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/cpp/m64.md	✅Succeeded

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[prmerger-automator]

PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.

[Court72]

@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]

Thank you for the code escape. Also appreciate you simplifying the path to files in the links and the HTML breaks :-)

[TylerMSFT]

#sign-off