Tighten wording in mutable data members

[Rageking8]

Add missing "non-reference" constraint, since mutable is not allowed on reference data members. The empty main function is removed as it does not serve any purpose.

[prmerger-automator]

@Rageking8 : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 3ba42ac:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
docs/cpp/mutable-data-members-cpp.md	⚠️Warning		Details

docs/cpp/mutable-data-members-cpp.md

• Line 4, Column 10: [Warning: ms-date-invalid - See documentation] Value out of range for 'ms.date': '03/14/2024'. If specified, must be a date in format M/D/YYYY, no later than today. If you don't specify ms.date, the date will be updated automatically.

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[Rageking8]

It is already 14 march in my timezone.

[TylerMSFT]

It is already 14 march in my timezone.

The system only cares about its timezone. We can either wait a day or back the date up 1 day.

[Rageking8]

I don't mind waiting till tomorrow. Will take note of the date in the future.

[Jak-MS]

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

[TylerMSFT]

Thank you.
#signoff

[TylerMSFT]

#sign-off