fix: Fix the error message regarding immutable attribute (probabl-ai#1108)

Fixing some outdated information from previous iteration.
`unfitted_estimator` and `fitted_estimator` are not available anymore
because we use the constructor directly.

Author: glemaitre

skore/src/skore/sklearn/_estimator/report.py

@@ -218,7 +218,7 @@ def X_train(self):
     def X_train(self, value):
         raise AttributeError(
             "The X_train attribute is immutable. "
-            "Please use the `from_unfitted_estimator` method to create a new report."
+            f"Call the constructor of {self.__class__.__name__} to create a new report."
         )
 
     @property
@@ -229,7 +229,7 @@ def y_train(self):
     def y_train(self, value):
         raise AttributeError(
             "The y_train attribute is immutable. "
-            "Please use the `from_unfitted_estimator` method to create a new report."
+            f"Call the constructor of {self.__class__.__name__} to create a new report."
         )
 
     @property