Doc: fix minor syntax error in example.

The CREATE TABLE option is GENERATED BY DEFAULT *AS* IDENTITY.

Per bug #18543 from Ondřej Navrátil.  Seems to have crept in
in a37bb7c13, so back-patch to v17 where that was added.

Discussion: https://postgr.es/m/18543-93c721689f9928e8@postgresql.org

doc/src/sgml/ddl.sgml

@@ -257,7 +257,7 @@ CREATE TABLE people (
    or alternatively
 <programlisting>
 CREATE TABLE people (
-    id bigint <emphasis>GENERATED BY DEFAULT IDENTITY</emphasis>,
+    id bigint <emphasis>GENERATED BY DEFAULT AS IDENTITY</emphasis>,
     ...,
 );
 </programlisting>