docs: fix gene count in AnnData getting started tutorial

[joncarse]

Summary

• Correct free-text gene count in the AnnData getting started tutorial from 13714 to 11505 so it matches adata output for the tutorial’s processed PBMC3k dataset.
• Verified by loading the tutorial h5ad via the notebook’s pooch.retrieve + anndata.read_h5ad path and checking adata.n_vars.

Test plan

• Confirm markdown after the first adata display says 2638 cells / 11505 genes
• Confirm notebook still contains stored outputs with 2638 × 11505 and no remaining 13714

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