Fix long-obsolete summary in mod_auth_digest.

<identifier>auth_digest_module</identifier>

<summary>

    <p>This module implements HTTP Digest Authentication. However, it

    has not been extensively tested and is therefore marked

    experimental.</p>

    <p>This module implements HTTP Digest Authentication

    (<a href="http://www.faqs.org/rfcs/rfc2617.html">RFC2617</a>), and

    provides a more secure alternative to <module>mod_auth_basic</module>.</p>

</summary>

<seealso><directive module="mod_authn_core">AuthName</directive></seealso>