Correct name of ComparableFromDifference in Generics details (#4547)

The name ComparableFromDifferenceFn comes from the next example. In this
example ComparableFromDifference is the name of the class that will be
implictly cast to a Facet matching Comparable.

docs/design/generics/details.md

@@ -1852,7 +1852,7 @@ class IntWrapper {
       return left.x - right.x;
     }
   }
-  impl as Comparable = ComparableFromDifferenceFn(IntWrapper);
+  impl as Comparable = ComparableFromDifference(IntWrapper);
 }
 ```