[Ada] Fix Warn_On_All_Unread_Out_Parameters not being properly tagged

[Pierre-Marie de Rodat via Gcc-patches]

This caused tools ingesting GNAT's output to mislabel these messages.

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

gcc/ada/

        * sem_warn.adb (Warn_On_Useless_Assignment): Fix insertion
        character.
diff --git a/gcc/ada/sem_warn.adb b/gcc/ada/sem_warn.adb
--- a/gcc/ada/sem_warn.adb
+++ b/gcc/ada/sem_warn.adb
@@ -4595,7 +4595,7 @@ package body Sem_Warn is
                         then
                            if Warn_On_All_Unread_Out_Parameters then
                               Error_Msg_NE
-                                ("?m?& modified by call, but value might not "
+                                ("?.o?& modified by call, but value might not "
                                  & "be referenced", LA, Ent);
                            end if;
                         else