Adjust is_error_code_enum and is_error_condition_enum syntax

[Rageking8]

Summary:

• Add missing class keyword before _Enum
• Remove unneeded indent
• Change class-key to struct as it's more accurate
  • One downside is that the page names contain the word "Class", and we can't change them without a redirect (which is undesirable)
  • https://github.com/microsoft/STL/blob/1f6e5b16ec02216665624c1e762f3732605cf2b4/stl/inc/system_error#L42-L43
  • https://github.com/microsoft/STL/blob/1f6e5b16ec02216665624c1e762f3732605cf2b4/stl/inc/system_error#L51-L52
  • https://eel.is/c++draft/system.error.syn
• Edit metadata

[prmerger-automator]

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

[learn-build-service-prod]

Learn Build status updates of commit db3990a:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/standard-library/is-error-code-enum-class.md	✅Succeeded
docs/standard-library/is-error-condition-enum-class.md	✅Succeeded

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[ttorble]

@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]

Good fix. Much thanks.

[TylerMSFT]

#sign-off