Document the "S" option for psql's \dn command in the psql help

This option was recently introduced, but the documentation in help.c
was not updated.

src/bin/psql/help.c

@@ -212,7 +212,7 @@ slashUsage(unsigned short int pager)
 	fprintf(output, _("  \\di[S+] [PATTERN]      list indexes\n"));
 	fprintf(output, _("  \\dl                    list large objects, same as \\lo_list\n"));
 	fprintf(output, _("  \\dL[S+] [PATTERN]      list procedural languages\n"));
-	fprintf(output, _("  \\dn[+]  [PATTERN]      list schemas\n"));
+	fprintf(output, _("  \\dn[S+] [PATTERN]      list schemas\n"));
 	fprintf(output, _("  \\do[S]  [PATTERN]      list operators\n"));
 	fprintf(output, _("  \\dp     [PATTERN]      list table, view, and sequence access privileges\n"));
 	fprintf(output, _("  \\drds [PATRN1 [PATRN2]] list per-database role settings\n"));