[1.8.x] Fixed typo in docs/ref/templates/builtins.txt

In addition to web links, ``urlize`` also converts email addresses into

``mailto:`` links. If ``value`` is

``"Send questions to foo@example.com"``, the output will be

``"Send questions to <a href="mailto:foo@example.com">foo@example</a>"``.

``"Send questions to <a href="mailto:foo@example.com">foo@example.com</a>"``.

The ``urlize`` filter also takes an optional parameter ``autoescape``. If

``autoescape`` is ``True``, the link text and URLs will be escaped using