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