Change self._num_features in ValueError message (backport #981) (#984)

* Correct `self._num_features` in ValueError (#981)

(cherry picked from commit 56bbed0)

* Update copyright in `base_kernel.py`

---------

Co-authored-by: Edoardo Altamura <[email protected]>

Author: mergify[bot]

qiskit_machine_learning/kernels/base_kernel.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2022, 2023.
+# (C) Copyright IBM 2022, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -116,7 +116,7 @@ def _validate_input(
                 raise ValueError(
                     f"x_vec and class feature map have incompatible dimensions.\n"
                     f"x_vec has {x_vec.shape[1]} dimensions, "
-                    f"but feature map has {self._feature_map.num_parameters}."
+                    f"but feature map has {self._num_features}."
                 ) from a_e
 
         if y_vec is not None: