Fix grammar in comment.
gcc/ada/ChangeLog:
* einfo.ads (Ghost_Assertion_Level): Fix comment.
-- Ghost_Assertion_Level
-- Assertion level associated with the declaration of the entity. Its
--- value is either Empty for non-ghost entities. Standard_Level_Default
--- for Ghost entities without an assertion level or a user defined
+-- value is either Empty for non-ghost entities, Standard_Level_Default
+-- for Ghost entities without an assertion level, or a user-defined
-- assertion level.
-- Has_Aliased_Components [implementation base type only]