Added an example of using join to extract just values

Also fixed the require of one of the examples so people following along at the REPL can just copy and paste the example.

Signed-off-by: Christian Romney <cromney@pointslope.com>

`join` is intended to remove one level of monadic context from a value, the type in Haskell is

```haskell
join :: Monad m => m (m a) -> m a
```
The intended usage is the following:

```clojure
(join (just (just 42)))
; #<Just [42]>
```

The recommended way to extract values from a Maybe instance is `from-maybe`, it'd be great if you changed `join` to `from-maybe`.