Fix documentation for memoized subject instances

* This is not a class handling *all* memoizations.
  Just on especific cases where the memoization semantics had been
  set-up via the memoizable gem.

lib/mutant/subject/method/instance.rb

@@ -27,7 +27,7 @@ module Mutant
           self
         end
 
-        # Mutator for memoized instance methods
+        # Mutator for memoizable memoized instance methods
         class Memoized < self
           include AST::Sexp