doc: include typo in 'unhandledRejection' example

Reintroduces an intentional typo in a process doc example.

Fixes: https://github.com/nodejs/node/issues/5644
PR-URL: https://github.com/nodejs/node/pull/5654


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>