Fix docstring of to_backend in backend_api.py (#8879)

The second method overloading of to_backend() has the wrong signature
in docstring

Signed-off-by: Yufeng Shi <[email protected]>

Author: YufengShi-dudu

exir/backend/backend_api.py

@@ -1,5 +1,6 @@
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 # All rights reserved.
+# Copyright 2025 Arm Limited and/or its affiliates.
 #
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
@@ -56,9 +57,9 @@ def to_backend(
      ) -> LoweredBackendModule:
 
      def to_backend(
-         graph_module: torch.fx.GraphModule,
-         partitioner: Type[TPartitioner],
-     ) -> torch.fx.GraphModule
+         edge_program: ExportedProgram,
+         partitioner: Partitioner,
+     ) -> ExportedProgram:
     """
     pass