Update DNS length comment.
Update DNS length comment.

Followup from https://github.com/letsencrypt/boulder/pull/1532/files#r54189791

patf/boulder,mehmooda/boulder,jfrazelle/boulder,letsencrypt/boulder,patf/boulder,benileo/boulder,ZCloud-Firstserver/boulder,jcjones/boulder,sophie-h/boulder,mehmooda/boulder,lucasallan/boulder,bmw/boulder,lucasallan/boulder,ZCloud-Firstserver/boulder,benileo/boulder,lucasallan/boulder,jfrazelle/boulder,ZCloud-Firstserver/boulder,letsencrypt/boulder,sophie-h/boulder,ibukanov/boulder,jfrazelle/boulder,mehmooda/boulder,ibukanov/boulder,ZCloud-Firstserver/boulder,jsha/boulder,jcjones/boulder,jsha/boulder,sophie-h/boulder,letsencrypt/boulder,jcjones/boulder,ibukanov/boulder,bmw/boulder,jsha/boulder,patf/boulder,jcjones/boulder,benileo/boulder,patf/boulder,jfrazelle/boulder,bmw/boulder,benileo/boulder,mehmooda/boulder