fix exception  message

Author: PhilippBach

doubleml/double_ml.py

@@ -1092,7 +1092,7 @@ def evaluate_learners(self, learners=None, metric=mean_squared_error):
 
         # check metric
         if not callable(metric):
-            raise TypeError('metric should be either a callable. '
+            raise TypeError('metric should be a callable. '
                             '%r was passed.' % metric)
 
         if all(learner in self.params_names for learner in learners):

doubleml/tests/test_doubleml_exceptions.py

@@ -808,7 +808,7 @@ def test_double_ml_exception_evaluate_learner():
 
     dml_irm_obj.fit()
 
-    msg = "metric should be either a callable. 'mse' was passed."
+    msg = "metric should be a callable. 'mse' was passed."
     with pytest.raises(TypeError, match=msg):
         dml_irm_obj.evaluate_learners(metric="mse")