Fix code block format in docs/plugins.rst

[mushitoriami]

On the current document, the bash command example for TOX_DISABLED_EXTERNAL_PLUGINS is rendered incorrectly.

This is because it uses Markdown-style code block (```bash) instead of reStructuredText syntax. This PR changes it to use the Sphinx .. code-block:: bash directive.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]