Fix Exception message for set_item call on FrozenDict

Previously it was set up to show `__setattr__`, likely due to a copy-paste error.
This is a minor fix as it just makes the error message correct.

Author: cartesian-plane

src/diffusers/configuration_utils.py

@@ -81,7 +81,7 @@ def __setattr__(self, name, value):
 
     def __setitem__(self, name, value):
         if hasattr(self, "__frozen") and self.__frozen:
-            raise Exception(f"You cannot use ``__setattr__`` on a {self.__class__.__name__} instance.")
+            raise Exception(f"You cannot use ``__setitem__`` on a {self.__class__.__name__} instance.")
         super().__setitem__(name, value)