Remove deprecated Course.Monad comment in readme

The reference to system-f#118 was because `Course.Monad` used to lack exercises since  [Course.Bind contained all of them](system-f@d63e288#diff-0a369498a5a8db3ac8fa606b544c9810R186). However, this was addressed in system-f#164 and system-f@9414bf2#diff-93667008909f8bff80964d12b2d3473b.

This PR updates the readme to remove the deprecated remark.

README.markdown

@@ -191,7 +191,7 @@ After this, the following progression of modules is recommended:
 * `Course.List`
 * `Course.Functor`
 * `Course.Applicative`
-* `Course.Monad` (please see [this issue](https://github.com/NICTA/course/issues/118))
+* `Course.Monad`
 * `Course.FileIO`
 * `Course.State`
 * `Course.StateT`