fix docstring for combine_first: returns a Dataset (#3683)

Author: David Caron

xarray/core/dataset.py

@@ -4152,7 +4152,7 @@ def combine_first(self, other: "Dataset") -> "Dataset":
 
         Returns
         -------
-        DataArray
+        Dataset
         """
         out = ops.fillna(self, other, join="outer", dataset_join="outer")
         return out