Fix typos in psconvert in the file figure.py

[yvonnefroehlich]

Description of proposed changes

This PR aims to fix typos in the API documentation of pygmt.Figure.psconvert in the file figure.py.

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.

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

[yvonnefroehlich]

This PR is more or less complete from my side. 

For the sake of consistency I want[ed] to add dots at the end of the bullet points in the section 'main features' of the API References. However, I could not figure out where / in which file I have to make the change. In the pygmt/doc/index.rst file, I only found:

.. _api:
API Reference
=============
.. automodule:: pygmt
.. currentmodule:: pygmt
Plotting

I would be happy if one could help me out 🙂 . If people agree this change makes sense, I can open a new PR or add this to this PR.

[seisman]

This PR is more or less complete from my side. 

For the sake of consistency I want[ed] to add dots at the end of the bullet points in the section 'main features' of the API References. However, I could not figure out where / in which file I have to make the change. In the pygmt/doc/index.rst file, I only found:

.. _api:
API Reference
=============
.. automodule:: pygmt
.. currentmodule:: pygmt
Plotting

I would be happy if one could help me out 🙂 . If people agree this change makes sense, I can open a new PR or add this to this PR.

They are here https://github.com/GenericMappingTools/pygmt/blob/main/pygmt/__init__.py.

[yvonnefroehlich]

They are here https://github.com/GenericMappingTools/pygmt/blob/main/pygmt/__init__.py.

Thanks @seisman! I have made the change in PR #2055.