Update DockPanel.md

[mael8]

Fixes

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

Wrongly suggests to close Page tag in the "Syntax" section

What is the new behavior?

Don't close the Page tag before the DockPanel sample

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [x] Based off latest main branch of toolkit
• [x] Tested code with current supported SDKs
• [ ] New component
  • [ ] Documentation has been added
  • [ ] Sample in sample app has been added
  • [ ] Analyzers are passing for documentation and samples
  • [ ] Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• [ ] Tests for the changes have been added (if applicable)
• [ ] Header has been added to all new source files
• [x] Contains NO breaking changes

Other information

[mael8]

@dotnet-policy-service agree