Fix old references from gmt to pygmt

[seisman]

Description of proposed changes

The project was renamed from from GMT/Python to PyGMT two years ago (see #263, #265, and #266).

In #264, we also corrected the references from

:meth:`~gmt.clib.Session.call_module`

to

:meth:`~pygmt.clib.Session.call_module`

Obviously, the replacement in #265 is not completed, and we still have some references
using the old package name.

Fixes #

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 docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version