Correct the Python Developer Guide markup style advice link

[hugovk]

Subject: Update link to Python devguide's advice on section markup

Feature or Bugfix

• Bugfix

Purpose

• The Python Developer's Guide has moved the recommendation on section headers to another page
• Let's update the link

Detail

• Change: https://devguide.python.org/documentation/style-guide/
• To: https://devguide.python.org/documentation/markup/#sections

Relates

• Reorganise the devguide to directories  python/devguide#901