pgsql: Add mention of checking <link> behavior of HISTORY.html to

Log Message:
-----------
Add mention of checking <link> behavior of HISTORY.html to release
checklist.

Modified Files:
--------------
pgsql/src/tools:
RELEASE_CHANGES (r1.72 -> r1.73)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/RELEASE_CHANGES.diff?r1=1.72&amp;r2=1.73)