Fix import in block editor’s readme example

[stokesman]

What?

Fixes imports in example.

Why?

Docs should not contain mistakes.

How?

• Removes WritingFlow import that was unused.
• Adds BlockCanvas import that was missing.

[github-actions]

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Co-authored-by: stokesman <presstoke@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[github-actions]

Flaky tests detected in 54f24e1.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/8884828783
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[Mamaduka]

Good catch! ✅