fix typo:CREATE POLICY username->username

Author: ChenHuajun

postgresql/doc/src/sgml/ddl.sgml

@@ -3122,7 +3122,7 @@ CREATE POLICY all_view ON passwd FOR SELECT USING (true);
 CREATE POLICY user_mod ON passwd FOR UPDATE
   USING (current_user = user_name)
   WITH CHECK (
-    current_user = username AND
+    current_user = user_name AND
     shell IN ('/bin/bash','/bin/sh','/bin/dash','/bin/zsh','/bin/tcsh')
   );