Fix ghostscript typo in environment.yml

[weiji14]

Description of proposed changes

Fix ghotscript -> ghostscript.

Patches typo in #3112.

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.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

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

• /format: automatically format and lint the code

[weiji14]

Do we actually need to pin ghostscript in environment.yml? This line was added in #3112, but we didn't set any pin for ghostscript before.

[seisman]

Better to pin ghostscript here. Otherwise, new developers may install a different gs version and the tests may fail due to minor changes in gs.