DOC: update the pandas.DataFrame.diff docstring

Author: adatasetaday

pandas/core/frame.py

@@ -4736,7 +4736,7 @@ def diff(self, periods=1, axis=0):
         axis : {0 or 'index', 1 or 'columns'}, default 0
             Take difference over rows (0) or columns (1).
 
-            .. versionadded:: 0.16.1
+            .. versionadded:: 0.16.1.
 
         Returns
         -------