Fix markup.

Doc/library/math.rst

@@ -300,7 +300,7 @@ The module also defines two mathematical constants:
    platform and libm implementation. It's usually :exc:`ValueError` for *EDOM*
    and :exc:`OverflowError` for errno *ERANGE*.
 
-   ..versionchanged:: 2.6
+   .. versionchanged:: 2.6
       In earlier versions of Python the outcome of an operation with NaN as
       input depended on platform and libm implementation.