Add "** DISPUTED **" to GHSA-6f39-fvhf-c6qr advisory

[jasnow]

Add "** DISPUTED **" to GHSA-6f39-fvhf-c6qr's Title and Description as I have seen in several other disputed advisories.

Why: See last comment in reference: https://bugs.ruby-lang.org/issues/13742

Note that this is not a vulnerability.
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-11465 is invalid.

Thanks.