Doc: fix minor oversight in ALTER DEFAULT PRIVILEGES ref page.

Since schemas have more than one kind of privilege, we should
use the synopsis form that shows the privilege being possibly
repeated.

Yugo Nagata

Discussion: https://postgr.es/m/20240424155052.7ac0d0773e4ae27ab723faea@sraoss.co.jp

doc/src/sgml/ref/alter_default_privileges.sgml

@@ -46,7 +46,8 @@ GRANT { USAGE | ALL [ PRIVILEGES ] }
     ON TYPES
     TO { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...] [ WITH GRANT OPTION ]
 
-GRANT { USAGE | CREATE | ALL [ PRIVILEGES ] }
+GRANT { { USAGE | CREATE }
+    [, ...] | ALL [ PRIVILEGES ] }
     ON SCHEMAS
     TO { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...] [ WITH GRANT OPTION ]
 
@@ -77,7 +78,8 @@ REVOKE [ GRANT OPTION FOR ]
     [ CASCADE | RESTRICT ]
 
 REVOKE [ GRANT OPTION FOR ]
-    { USAGE | CREATE | ALL [ PRIVILEGES ] }
+    { { USAGE | CREATE }
+    [, ...] | ALL [ PRIVILEGES ] }
     ON SCHEMAS
     FROM { [ GROUP ] <replaceable class="parameter">role_name</replaceable> | PUBLIC } [, ...]
     [ CASCADE | RESTRICT ]