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

diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 513112a216d96b6a293f712b453b3f94bf411858..b4648321dc4c29cdae4edd01e42b26b0954cdfa1 100644 (file)
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -956,7 +956,7 @@ CREATE TABLE tree (
 );
 
     A top-level node would have NULL parent_id,
-    but non-NULL parent_id entries would be
+    while non-NULL parent_id entries would be
     constrained to reference valid rows of the table.