Fix docstring

quantile_forest/_quantile_forest.py

@@ -18,7 +18,7 @@ class calls the ``fit`` method of the ``ForestRegressor`` and creates a
   that extends the extremely randomized trees ``ExtraTreesRegressor`` as the
   estimator implementation.
 
-Single and multi-output problems are both handled.
+Single- and multi-output problems are both handled.
 """
 
 import random