[Ada] Fix typos in comment about expansion of attribute Image

[Pierre-Marie de Rodat]

Minor fix in comment; behaviour is unaffected. Spotted while examining
the expansion of attribute Image in GNAT so it can be better supported
in GNATprove.

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

gcc/ada/

        * exp_imgv.adb (Expand_User_Defined_Enumeration_Image): Fix
        typos.
diff --git a/gcc/ada/exp_imgv.adb b/gcc/ada/exp_imgv.adb
--- a/gcc/ada/exp_imgv.adb
+++ b/gcc/ada/exp_imgv.adb
@@ -952,7 +952,7 @@ package body Exp_Imgv is
          end;
 
          --  Generate:
-         --    S4 : String renames call_put_enumS (S2 .. S3 - 1);
+         --    P4 : String renames call_put_enumS (P2 .. P3 - 1);
 
          declare
             Sub_Node : constant Node_Id := New_Op_Node (N_Op_Subtract, Loc);