fix MultiOutputEstimator fit return docstring (scikit-learn#7150)

Author: nelson-liu

sklearn/multioutput.py

@@ -60,9 +60,11 @@ def fit(self, X, y, sample_weight=None):
             Sample weights. If None, then samples are equally weighted.
             Only supported if the underlying regressor supports sample
             weights.
+
         Returns
         -------
-        self
+        self : object
+            Returns self.
         """
 
         if not hasattr(self.estimator, "fit"):