Remove the "GitHub Actions" subsection heading from MAINTENANCE.md

[seisman]

Description of proposed changes

• The only CI service we're using is "GitHub Actions", so no need to have the "GitHub Actions" subsection
• Fix a typo in markdown foramt

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[weiji14]

Looks good, but please change the title to something else. There's a lot of 'tiny' changes recently. Maybe 'Remove GitHub Actions subsection in MAINTENANCE.md'?

[michaelgrund]

Looks fine to me!