added tree of type "regressor" to the docstring of exportviz (scikit-learn#9530)

Author: lucianoviola

sklearn/tree/export.py

@@ -92,7 +92,7 @@ def export_graphviz(decision_tree, out_file=SENTINEL, max_depth=None,
 
     Parameters
     ----------
-    decision_tree : decision tree classifier
+    decision_tree : decision tree regressor or classifier
         The decision tree to be exported to GraphViz.
 
     out_file : file object or string, optional (default='tree.dot')