DOC: Fix warning with Sphinx 8

The `metpy` package on these modules is redundant since we've already
set the current module to `metpy` above.

docs/api/index.rst

@@ -9,14 +9,14 @@ Reference Guide
 .. autosummary::
  :toctree: generated/
 
- metpy.constants
- metpy.units
- metpy.io
- metpy.calc
- metpy.plots
- metpy.plots.ctables
- metpy.interpolate
- metpy.xarray
+ constants
+ units
+ io
+ calc
+ plots
+ plots.ctables
+ interpolate
+ xarray
 
 .. toctree::
  :maxdepth: 1