Fix wrong error message in nonlinear controller

Replace `self.y_s.shape` with `self.y_r.shape` in the `ValueError`
message of the `set_output_setpoint` method, as it was a typo.

Author: pavelacamposp

direct_data_driven_mpc/nonlinear_data_driven_mpc_controller.py

@@ -1385,7 +1385,7 @@ def set_output_setpoint(self, y_r: np.ndarray) -> None:
         if y_r.shape != self.y_r.shape:
             raise ValueError(
                 "Incorrect dimensions. `y_r` must have shape "
-                f"{self.y_s.shape}, but got {y_r.shape} instead."
+                f"{self.y_r.shape}, but got {y_r.shape} instead."
             )
 
         # Update output setpoint and its parameter value