Doc: trivial wording adjustment.

Improve self-referential foreign key example, per suggestion
from David Johnston.

Discussion: https://postgr.es/m/CAKFQuwZTke7+HUn4YUGqu2+gAPi4Cy18TXMrg_Z5nADkxfPNMw@mail.gmail.com

doc/src/sgml/ddl.sgml

@@ -956,7 +956,7 @@ CREATE TABLE tree (
 );
 </programlisting>
     A top-level node would have NULL <structfield>parent_id</structfield>,
-    but non-NULL <structfield>parent_id</structfield> entries would be
+    while non-NULL <structfield>parent_id</structfield> entries would be
     constrained to reference valid rows of the table.
    </para>