Fix documentation referring to removed as_gpu_matrix method.

[bdice]

Description

This fixes outdated documentation that refers to the as_gpu_matrix method, which was removed.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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Codecov Report

Base: 87.40% // Head: 88.12% // Increases project coverage by +0.71% 🎉

Coverage data is based on head (396cf7b) compared to base (f72c4ce).
Patch coverage: 87.89% of modified lines in pull request are covered.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #11937      +/-   ##
================================================
+ Coverage         87.40%   88.12%   +0.71%     
================================================
  Files               133      133              
  Lines             21833    21897      +64     
================================================
+ Hits              19084    19297     +213     
+ Misses             2749     2600     -149     

Impacted Files	Coverage Δ
python/cudf/cudf/core/udf/__init__.py	97.05% <ø> (+47.05%)	⬆️
python/cudf/cudf/io/orc.py	92.94% <ø> (-0.09%)	⬇️
...thon/dask_cudf/dask_cudf/tests/test_distributed.py	18.86% <ø> (+4.94%)	⬆️
python/cudf/cudf/core/_base_index.py	82.20% <43.75%> (-3.35%)	⬇️
python/cudf/cudf/io/text.py	91.66% <66.66%> (-8.34%)	⬇️
python/strings_udf/strings_udf/__init__.py	86.27% <76.00%> (-10.61%)	⬇️
python/cudf/cudf/core/index.py	92.96% <95.71%> (+0.33%)	⬆️
python/cudf/cudf/__init__.py	90.69% <100.00%> (ø)
python/cudf/cudf/core/column/categorical.py	89.34% <100.00%> (ø)
python/cudf/cudf/core/groupby/groupby.py	91.51% <100.00%> (+0.29%)	⬆️
... and 13 more

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[bdice]

@gpucibot merge