Figure.meca: Fix typo pricipal_axis -> principal_axis

[weiji14]

Description of proposed changes

Corrects a typo in Figure.meca that was introduced in #2576 and affects PyGMT v0.10.0, resulting in an error like GMTInvalidInput: Invalid convention 'pricipal_axis' being raised.

Reported by @yasuit21 at https://twitter.com/yasuit21/status/1724513532312698953#m

Patches #2576

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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