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 79.93%. Comparing base (c52da39) to head (72fab54).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4202   +/-   ##
=======================================
  Coverage   79.93%   79.93%           
=======================================
  Files         121      121           
  Lines       12958    12958           
=======================================
  Hits        10358    10358           
  Misses       2600     2600           

Flag	Coverage Δ
hatch-test.low-vers	79.06% <ø> (ø)
hatch-test.pre	79.88% <ø> (ø)

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)

[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

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 1.12.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 8a2be721bac37268eedb6cb625666fe35dc5f930

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am "Backport PR #4202: docs: Remove duplicated 'layer' parameter from PCA documentation"

4. Push to a named branch:

git push YOURFORK 1.12.x:auto-backport-of-pr-4202-on-1.12.x

5. Create a PR against branch 1.12.x, I would have named this PR:

"Backport PR #4202 on branch 1.12.x (docs: Remove duplicated 'layer' parameter from PCA documentation)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.