Doc: fix minor bug in CREATE TABLE example.

One example in create_table.sgml claimed to be showing table constraint
syntax, but it was really column constraint syntax due to the omission
of a comma.  This is both wrong and confusing, so fix it in all
supported branches.

Per report from neil@postgrescompare.com.

Discussion: https://postgr.es/m/151871659877.1393.2431103178451978795@wrigleys.postgresql.org

doc/src/sgml/ref/create_table.sgml

@@ -1531,7 +1531,7 @@ CREATE TABLE distributors (
 <programlisting>
 CREATE TABLE distributors (
     did     integer,
-    name    varchar(40)
+    name    varchar(40),
     CONSTRAINT con1 CHECK (did &gt; 100 AND name &lt;&gt; '')
 );
 </programlisting>