docs: Markdown examples in controls/markdown.md

[7576457]

Description

This PR fixes the examples in the documentation at: https://docs.flet.dev/controls/markdown/#basic-example

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I signed the CLA.
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing tests pass locally with my changes
• I have made corresponding changes to the documentation (if applicable)

Screenshots

Summary by Sourcery

Fix Markdown control examples and associated documentation assets to use correct fenced code block syntax and media path references.

Documentation:

• Correct Markdown fenced code block syntax in example files to properly specify languages and rendering.
• Update Markdown documentation image reference to use the correct media path for the code syntax highlight example.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

[FeodorFitsner]

Never seen such a syntax. Is ~~~ required when language has been specified?