Fix tiny typo in documentation of "G" of "gmtwhich"

[yvonnefroehlich]

Description of proposed changes

Following the review in GenericMappingTools/pygmt#2519 (comment), this PR aims to fix a tiny typo in the documentation of G of gmtwhich.

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[anbj]

Looks good!