Add str as a valid option for the save argument in scanpy.pl.rank_genes_groups

[JoshLoecker]

This is a very small pull request to add str to the possible arguments for saving a figure from scanpy.pl.rank_genes_groups. This addition matches other save= arguments, such as from scanpy.plotting.highly_variable_genes, sc.plotting.pca_variance_ratio, and scanpy.plotting.umap

I have not included tests or release notes due to the single-line change nature of this pull request

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.87%. Comparing base (3ba3f46) to head (10c4e0e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3076   +/-   ##
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  Coverage   75.87%   75.87%           
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  Files         110      110           
  Lines       12533    12533           
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  Hits         9509     9509           
  Misses       3024     3024           

Files	Coverage Δ
scanpy/plotting/_tools/__init__.py	77.28% <ø> (ø)