Fix error message which would print the article "the" twice. (#764)

In all cases, `arg_description` already contains the article "the". The error message would then contain "the" twice, for instance "Intead, the the targets should...".

Also fixed some formatting issues caught by lint.

Author: hertschuh

keras_core/trainers/epoch_iterator.py

@@ -220,7 +220,7 @@ def num_batches(self):
 def raise_unsupported_arg(arg_name, arg_description, input_type):
     raise ValueError(
         f"When providing `x` as a {input_type}, `{arg_name}` "
-        f"should not be passed. Instead, the {arg_description} should "
+        f"should not be passed. Instead, {arg_description} should "
         f"be included as part of the {input_type}."
     )