DOC: Fix parameter in reindex_axis docstring

Author: kyleam

pandas/core/generic.py

@@ -1663,7 +1663,7 @@ def _reindex_multi(self, axes, copy, fill_value):
 
         Parameters
         ----------
-        index : array-like, optional
+        labels : array-like
             New labels / index to conform to. Preferably an Index object to
             avoid duplicating data
         axis : %(axes_single_arg)s