Figure.inset: Fix typos in docstring of "box" parameter

[yvonnefroehlich]

Description of proposed changes

This PR aims to fix a few typos in the docstring of the box parameter of Figure.inset.

Fixes #

Reminders

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• 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.
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• If adding new functionality, add an example to docstrings or tutorials.
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