Clarify that abstract types are supported by loads:

cjoudrey · cjoudrey · commit f9ccb717a288 · 2018-09-05T15:50:50.000-04:00
diff --git a/guides/mutations/mutation_classes.md b/guides/mutations/mutation_classes.md
@@ -153,3 +153,5 @@ class Mutations::AddStar < Mutations::BaseMutation
   end
 end
 ```
+
+In the above examples, `loads:` is provided a concrete type, but it also supports abstract types (i.e. interfaces and unions).

-Original file line number
+Diff line change
   end
 end
 ```
++
 +In the above examples, `loads:` is provided a concrete type, but it also supports abstract types (i.e. interfaces and unions).