Fixed #17309 -- Typo in the IGNORABLE_404_URLS example.

    import re

    IGNORABLE_404_URLS = (

        re.compile(r'^/apple-touch-icon.*\.png$'),

        re.compile(r'^/favicon\.ico$),

        re.compile(r'^/robots\.txt$),

        re.compile(r'^/favicon\.ico$'),

        re.compile(r'^/robots\.txt$'),

    )

(Note that these are regular expressions, so we put a backslash in front of