Fix typo in documentation of toast storage

Author: Nawaz Ahmed
Discussion: https://postgr.es/m/154319327168.1315.1846953598601966513@wrigleys.postgresql.org

doc/src/sgml/storage.sgml

@@ -237,7 +237,7 @@ segments as well, though this is unlikely to happen in practice.
 <para>
 A table that has columns with potentially large entries will have an
 associated <firstterm>TOAST</firstterm> table, which is used for out-of-line storage of
-field values that are too large to keep in the table rows proper.
+field values that are too large to keep in the table rows properly.
 <structname>pg_class</structname>.<structfield>reltoastrelid</structfield> links from a table to
 its <acronym>TOAST</acronym> table, if any.
 See <xref linkend="storage-toast"/> for more information.