Update text symbol gallery example

[michaelgrund]

Description of proposed changes

According to a recent forum post, here's an update for the text symbol gallery example. Since it is not fully clear from the current example how to change the outline color of the symbols, for the last symbol now the outline color is changed and the docstring is slightly adjusted.

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.

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

[seisman]

Looks good to me.

[willschlitzer]

Looks good!