docs: mistune: Remove incorrect claim about fenced code blocks

[vincentbernat]

The guide claimed Mistune only understands three backticks, but it actually supports 4-space indented code blocks too.

import mistune
md = mistune.create_markdown(escape=True)
md('    indented code block\n    second line\n')
md('```\nbacktick fenced\n```\n')

Both render as <pre><code>...</code></pre>.

Checklist

• All new and existing tests are passing
• (If adding features:) I have added tests to cover my changes
• (If docs changes needed:) I have updated the documentation accordingly.
• I have added an entry to CHANGES.rst because this is a user-facing change or an important bugfix
• I have written proper commit message(s)