Tiny improvements for the multi-parameter symbols example

[core-man]

Description of proposed changes

Tiny improvements for the comments in the gallery example for the multi-parameter symbols

Related to #983 and #996.

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

[michaelgrund]

Since there were only minor fixes, this PR looks good to me.

[seisman]

@core-man It looks good. Could you also update the docstrings to make lines shorter than 80 chars?

[core-man]

@core-man It looks good. Could you also update the docstrings to make lines shorter than 80 chars?

done in fad6ba5.