Correct the type of the `MergeRequestResolver`.

This was overridden in the fields where this resolver was used. So
this change is purely cosmetic.

app/graphql/resolvers/merge_request_resolver.rb

@@ -2,7 +2,7 @@ module Resolvers
   class MergeRequestResolver < BaseResolver
     prepend FullPathResolver
-    type Types::ProjectType, null: true
+    type Types::MergeRequestType, null: true
     argument :iid, GraphQL::ID_TYPE,
              required: true,