fix highlight in the datetime example

Author: ezio-melotti

Doc/tutorial/stdlib.rst

@@ -181,15 +181,15 @@ focus of the implementation is on efficient member extraction for output
 formatting and manipulation.  The module also supports objects that are timezone
 aware. ::
 
-   # dates are easily constructed and formatted
+   >>> # dates are easily constructed and formatted
    >>> from datetime import date
    >>> now = date.today()
    >>> now
    datetime.date(2003, 12, 2)
    >>> now.strftime("%m-%d-%y. %d %b %Y is a %A on the %d day of %B.")
    '12-02-03. 02 Dec 2003 is a Tuesday on the 02 day of December.'
 
-   # dates support calendar arithmetic
+   >>> # dates support calendar arithmetic
    >>> birthday = date(1964, 7, 31)
    >>> age = now - birthday
    >>> age.days