Minor Dataset.map docstr clarification (#6595)

The summary line in the docstr for `Dataset.map()` says that it operates on variables in the dataset. This is a very minor update clarifying that it operates on *data* variables, as opposed to all variables (data + coord) in the Dataset.

xarray/core/dataset.py

@@ -5308,7 +5308,7 @@ def map(
         args: Iterable[Any] = (),
         **kwargs: Any,
     ) -> Dataset:
-        """Apply a function to each variable in this dataset
+        """Apply a function to each data variable in this dataset
 
         Parameters
         ----------