Fix incorrect link in v10 release notes.

As noted by M. Justin.

Also, to keep the HEAD and REL_10 versions of release-10.sgml in sync,
back-patch the effects of c29c57890 on that file.  We have a bigger
problem there though :-(

Discussion: https://postgr.es/m/CALtA7pmsQyTTD3fC2rmfUWgfivv5sCJJ84PHY0F_5t_SRc07Qg@mail.gmail.com
Discussion: https://postgr.es/m/6d137bd0-eef6-1d91-d9b8-1a5e9195a899@2ndquadrant.com
