Fix case sensitivity in Container Apps extension command

[YusukeTobo]

The extension name is written entirely in lowercase. ContainerApp resulted in the following error.

az extension add -n ContainerApp
No extension found with name 'ContainerApp'

Use --debug for more information

[prmerger-automator]

@YusukeTobo : 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 4f9d5c7:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/container-apps/deploy-express-cli.md	✅Succeeded

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[v-regandowner]

@craigshoemaker

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