Replace "gravity" by "gravitational" in documentation

[birocoles]

The functions point_mass_gravity, prism_gravity and tesseroid_gravity compute components of the gravitational field (and not the gravity field) produced by point massas, rectangular prisms and tesseroids, respectively. Hence, I have replaced the term "gravity" by "gravitational" in the documentation of these functions.

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 adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[leouieda]

👍 thanks, @birocoles! Sorry for the trouble. Keep them coming!

[welcome]

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