Doc: Fix link to fillfactor reloption.

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 115464bb.

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