Update axis label explanation in frames tutorial

[willschlitzer]

This improves the explanation of axis labeling in the frame.py tutorial.

Fixes #815

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[seisman]

Great!