[Ada] Fix typo in comment related to derived discriminated types

[Pierre-Marie de Rodat]

Minor typo; found while fixing handling of tagged types in GNATprove.

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

gcc/ada/

        * exp_util.ads (Map_Types): Fix typo.
diff --git a/gcc/ada/exp_util.ads b/gcc/ada/exp_util.ads
--- a/gcc/ada/exp_util.ads
+++ b/gcc/ada/exp_util.ads
@@ -915,7 +915,7 @@ package Exp_Util is
    --  Establish the following mapping between the attributes of tagged parent
    --  type Parent_Type and tagged derived type Derived_Type.
    --
-   --    * Map each discriminant of Parent_Type to ether the corresponding
+   --    * Map each discriminant of Parent_Type to either the corresponding
    --      discriminant of Derived_Type or come constraint.
 
    --    * Map each primitive operation of Parent_Type to the corresponding