docs: Remove duplicated 'layer' parameter from PCA documentation

[AlbertoFabbri93]

Fix small documentation error: the 'layer' parameter is documented twice for the PCA preprocessing function.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.92%. Comparing base (7df9ad6) to head (7583dfb).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4202   +/-   ##
=======================================
  Coverage   79.91%   79.92%           
=======================================
  Files         121      121           
  Lines       12949    12949           
=======================================
+ Hits        10348    10349    +1     
+ Misses       2601     2600    -1     

Flag	Coverage Δ
hatch-test.low-vers	79.04% <ø> (ø)
hatch-test.pre	79.87% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/preprocessing/_pca/__init__.py	91.92% <ø> (ø)

... and 1 file with indirect coverage changes

[Zethson]

Great! If you'd like, you could add a small release note to be credited later when we make a release. Otherwise, I'll get this in as is.

[AlbertoFabbri93]

Nah, it's okay. You can merge as is.

[Zethson]

generating indices... /home/docs/checkouts/readthedocs.org/user_builds/icb-scanpy/checkouts/4202/docs/release-notes/1.6.0.md:44: WARNING: unknown document: 'anndata:concatenation' [ref.doc]

shouldn't be your fault. I'll take care of this soon