Quickfix plot docstring to have label (l)

[weiji14]

Description of proposed changes

The code that sets label as an alias for l in plot is in the right place, but the docstring was accidentally placed under grdcontour instead of plot.

Note to future self/contributor: Wait for GenericMappingTools/gmt#1838 to actually use -l in grdcontour and contour.

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.