docs: bump tutorials submodule to pull in network label fix

[PauBadiaM]

Summary

Bumps the docs/notebooks submodule (scverse/decoupler-tutorials) from 6ca80f0 to 21c4b9a, pulling the merged tutorial fixes into the published docs.

The tutorials PR (scverse/decoupler-tutorials#1) addresses #331 — the dc.pl.network plot in the RNA enrichment vignettes was labeled "Gene expression" even though the input is a DESeq2 contrast statistic (t-values). It now:

• passes t_label="Gene-level statistic" to dc.pl.network and updates the surrounding markdown,
• migrates pyDESeq2 from the deprecated design_factors to the formulaic design argument,
• regenerates all outputs (warning-free; scanpy bumped to 1.12.1 to drop an anndata.obs_vector FutureWarning).

Merging this rebuilds the docs on Read the Docs (which fetches submodules) with the corrected tutorials.

Closes #331

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.01%. Comparing base (d06c042) to head (eb73279).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #334   +/-   ##
=======================================
  Coverage   94.01%   94.01%           
=======================================
  Files          78       78           
  Lines        4078     4078           
=======================================
  Hits         3834     3834           
  Misses        244      244           

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