Better example for os.system(): do not change the system time.

Doc/tutorial/stdlib.rst

@@ -14,11 +14,11 @@ The :mod:`os` module provides dozens of functions for interacting with the
 operating system::
 
    >>> import os
-   >>> os.system('time 0:02')
-   0
    >>> os.getcwd()      # Return the current working directory
    'C:\\Python31'
-   >>> os.chdir('/server/accesslogs')
+   >>> os.chdir('/server/accesslogs')   # Change current working directory
+   >>> os.system('mkdir today')   # Run the command mkdir in the system shell
+   0
 
 Be sure to use the ``import os`` style instead of ``from os import *``.  This
 will keep :func:`os.open` from shadowing the built-in :func:`open` function which