Doc: fix backwards description of visibility map's all-frozen data.

Thinko in commit a892234f8.

Vik Fearing

Discussion: https://postgr.es/m/b6aaa23d-e26f-6404-a30d-e89431492d5d@2ndquadrant.com

doc/src/sgml/storage.sgml

@@ -624,7 +624,7 @@ can be used to examine the information stored in free space maps.
 Each heap relation has a Visibility Map
 (VM) to keep track of which pages contain only tuples that are known to be
 visible to all active transactions; it also keeps track of which pages contain
-only unfrozen tuples.  It's stored
+only frozen tuples.  It's stored
 alongside the main relation data in a separate relation fork, named after the
 filenode number of the relation, plus a <literal>_vm</> suffix. For example,
 if the filenode of a relation is 12345, the VM is stored in a file called