Fix constructor_destructuring docs example to alert a defined value

The last 2 examples in the [Destructuring Assignment](http://coffeescript.org/#destructuring) section of the docs both call `run: contents.join("")` but the last example doesn't include any variables named `contents` (copy-paste error).

I only started coffee-script yesterday so if this example is trying to demonstrate something else or would be better off with <code>code_for('constructor_destructuring', <strong>executable=false</strong>)</code>, let me know!

Thanks for the language!  :smile: