updates "removing a deployed app documentation" in readme.md

The current instructions didn't tell the user to remove the PORT. As a result this could happen:

```
remote: -----> Deploying node-js-sample ...
remote: Error: No such container: 3475243bbebb
remote:
remote: 2013/08/03 14:33:28 Error: Error starting container dd04f775f98f: Port already in use: 49154
remote:       ERROR: failed on rev a3ce2b05693a21904a8a5c5f4d2bd3c2373faef1 - push denied
```