[DOC] Fix links of cve [ci skip]
`http://www.cve.mitre.org/` seem to move `http://cve.mitre.org/`.
I can't access `http://www.cve.mitre.org/`.
[Fix GH-1857]
From: willnet <netwillnet@gmail.com>
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63166 b2dd03c8-39d4-4d8f-98ff-823fe69b080e