[Ada] Fix reference to SPARK RM rule in comment

[Pierre-Marie de Rodat]

Adding rule about a user-defined equality operation as SPARK RM
6.1.4(11) is changing the numeration of previous rules (11,12).

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_prag.adb (Check_Mode_Restriction_In_Enclosing_Context):
        Fix reference to SPARK RM rule number.
diff --git a/gcc/ada/sem_prag.adb b/gcc/ada/sem_prag.adb
--- a/gcc/ada/sem_prag.adb
+++ b/gcc/ada/sem_prag.adb
@@ -2650,7 +2650,7 @@ package body Sem_Prag is
 
                   --  The item is classified as In_Out or Output but appears as
                   --  an Input in an enclosing subprogram or task unit (SPARK
-                  --  RM 6.1.4(12)).
+                  --  RM 6.1.4(13)).
 
                   if Appears_In (Inputs, Item_Id)
                     and then not Appears_In (Outputs, Item_Id)