[Ada] Adjust reference to renumbered SPARK RM rule

Rules in SPARK RM 7.1.3 were renumbered when adding support for tasking,
but references in the comments were not updated. Now fixed.

2018-07-16  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_ch3.adb (Process_Discriminants): Adjust reference to the SPARM RM
	rule.

From-SVN: r262710

gcc/ada/ChangeLog

+2018-07-16  Piotr Trojanek  <trojanek@adacore.com>
+
+	* sem_ch3.adb (Process_Discriminants): Adjust reference to the SPARM RM
+	rule.
+
 2018-07-16  Arnaud Charlet  <charlet@adacore.com>
 
 	* adaint.c (__gnat_set_file_time_name): Code clean up.

gcc/ada/sem_ch3.adb

@@ -20125,7 +20125,7 @@ package body Sem_Ch3 is
             end if;
          end if;
 
-         --  A discriminant cannot be effectively volatile (SPARK RM 7.1.3(6)).
+         --  A discriminant cannot be effectively volatile (SPARK RM 7.1.3(4)).
          --  This check is relevant only when SPARK_Mode is on as it is not a
          --  standard Ada legality rule.