Doc: Fix link to fillfactor reloption.

Fix a link from the "Heap-Only Tuples" documentation section.
Previously, its "fillfactor" link pointed to the "CREATE TABLE"
command's documentation.  Now the link directly points to the fillfactor
storage parameter documentation (which is about half way into the
"CREATE TABLE" sect1).

Oversight in commit 115464b.

Backpatch: 12-, the first version with a usable reloption link.

Author: petergeoghegan

doc/src/sgml/storage.sgml

@@ -1136,7 +1136,7 @@ data. Empty in ordinary tables.</entry>
   if columns used by indexes are not updated.  You can
   increase the likelihood of sufficient page space for
   <acronym>HOT</acronym> updates by decreasing a table's <link
-  linkend="sql-createtable"><literal>fillfactor</literal></link>.
+  linkend="reloption-fillfactor"><literal>fillfactor</literal></link>.
   If you don't, <acronym>HOT</acronym> updates will still happen because
   new rows will naturally migrate to new pages and existing pages with
   sufficient free space for new row versions.  The system view <link