DOC: fix copy-paste bug in PyArrow storage example in text.rst

[rishitoshsingh]

Description

The "PyArrow storage with pd.NA values" example in text.rst used
storage="python"; identical to the preceding "Python storage" example
and contradicting the section heading/text, which claims a PyArrow-backed
array. Fixed to storage="pyarrow".
No issue filed; this is a trivial one-word typo fix, which the
contributing guide exempts from requiring an associated issue.

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• I have reviewed and followed all the contribution guidelines
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

[Copilot]

Pull request overview

Fixes a copy/paste error in the user guide’s “PyArrow storage with pd.NA values” example so it actually constructs a PyArrow-backed StringDtype (matching the section heading and narrative).

Changes:

• Update the example pd.StringDtype(...) to use storage="pyarrow" instead of storage="python" in doc/source/user_guide/text.rst.

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[jbrockmendel]

thanks @rishitoshsingh