Update _data.py (#1101)

map_variable: minor technical correction in doc

Author: ddkohler

WrightTools/data/_data.py

@@ -1214,7 +1214,7 @@ def map_variable(
             between.
         input_units : str (optional)
             The units of the new points. Default is same, which assumes
-            the new points have the same units as the axis.
+            the new points have the same units as the variable.
         name : string (optional)
             The name of the new data object. If None, generated from
             natural_name. Default is None.