Highlight arguments for "style" attribute of "pen" parameter in Gallery example "Line styles"

[yvonnefroehlich]

Description of proposed changes

This PR adds highlighting for the arguments allowed for the style attribute of the pen parameter in the Gallery example Line styles.

Preview: https://pygmt-dev--2520.org.readthedocs.build/en/2520/gallery/lines/linestyles.html

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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