docs: fix --is-performatted typo in prepare_hidden_states Usage

[imitater-dou]

Summary

Fix a one-character typo in the module Usage docstring of scripts/prepare_hidden_states.py: --is-performatted → --is-preformatted.

The argparse flag is already --is-preformatted, and the prose line in the same docstring (add --is-preformatted) plus docs/basic_usage/data_preparation.md already use the correct spelling. Only the example command line had the typo.

Repro

# on main (bc5d8451)
rg -n 'is-performatted|is-preformatted' scripts/prepare_hidden_states.py
# Usage example shows --is-performatted (typo)
# argparse defines --is-preformatted; prose already says --is-preformatted

Test plan

• Confirmed single-line docstring change only
• Verified argparse / docs already correct; no behavioral change