Update fixture import and docstring in test_filter1d.py

[willschlitzer]

I forgot to change the docstring in fixture_table() in test_filter1d.py from whatever grid loading function I copied it from. This updates the docstring to accurately describe the function.

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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