Fix Comonad example

- Current example doesn't run
- `extract` is supposed to be a function
- `this.v` is not a thing
- Unfortunately, I'm not sure if it's possible to do this with arrow
functions, as they'll bind the current `this` scope, which will be
`CoIdentity`'s current scope instead of the actual returned object..

readme.md

@@ -441,8 +441,8 @@ compose(foo, identity) ≍ compose(identity, foo) ≍ foo
 ```js
 let CoIdentity = v => ({
     val: v,
-    extract: this.v,
+    extract: function () { return this.val },
-    extend: f => CoIdentity(f(this))
+    extend: function (f) { return CoIdentity(f(this)) }
 })
 ```