Update parameter 'uncertaintycolor' to 'uncertaintyfill' in example 'velo_arrow_ellipse.py'

[yvonnefroehlich]

Description of proposed changes

This PR aims to rename 'uncertaintycolor' to 'uncertaintyfill' in the gallery example Velocity arrows and confidence ellipses.

Related to: #2206 and #2201 (comment)

Adresses: #1617

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.

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