DOC: Fix copy-pasted name in .day_name docstring

Author: Peque

pandas/core/arrays/datetimes.py

@@ -826,7 +826,7 @@ def day_name(self, locale=None):
 
         Returns
         -------
-        month_names : Index
+        day_names : Index
             Index of day names
 
         .. versionadded:: 0.23.0