consistent usage of self-closing tags for components in all examples on this page.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Some of the code examples on this page are using self-closing tags for the same components while others are not.

Issue Number: N/A

What is the new behavior?

All the examples are using self-closing tags for the components.

Does this PR introduce a breaking change?

• Yes
• No

Other information