Added an example that uses signal.alarm() to time out an os.open() that

Added an example that uses signal.alarm() to time out an os.open() that
    takes too long. This example relies on the fact that raising an
    exception in a signal handler causes the exception to be re-raised
    when the main line of the program resumes execution.  Is this guaranteed
    in CPython, or is this something that just happens to work by accident?

Also fixed a typo.