fix: correct misleading ValueError message in Attention._call_

[Dhakshin2007]

Bug Fix: Incorrect ValueError message in Attention.__call__

Problem

In gemma/gm/nn/_modules.py, the ValueError raised when attn_type is neither AttentionType.LOCAL_SLIDING nor AttentionType.GLOBAL contains a misleading error message:

# Before (buggy)
raise ValueError(
    'Attn_type must be either AttentionType.GLOBAL or'
    f' AttentionType.GLOBAL not {self.attn_type}'
)

Both sides of "or" show AttentionType.GLOBAL — this is a copy-paste error. The first value should be AttentionType.LOCAL_SLIDING.

Fix

# After (fixed)
raise ValueError(
    'Attn_type must be either AttentionType.LOCAL_SLIDING or'
    f' AttentionType.GLOBAL not {self.attn_type}'
)

Impact

This does not affect runtime behavior (the condition itself is correct), but the misleading error message makes debugging difficult for users who encounter unsupported attention types.