Correct docs for "wedge" (w) symbol in gallery example "Multi-parameter symbols"

[yvonnefroehlich]

Description of proposed changes

For "pie wedges" (w) users have to state the diameter not the radius. This PR corrects the corresponding gallery example Multi-parameter symbols.

Please see also the upstream GMT documentation: https://docs.generic-mapping-tools.org/dev/plot.html#id2

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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