Doc: trivial wording adjustment.
Improve self-referential foreign key example, per suggestion
from David Johnston.
Discussion:
@@ -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>
