Fix missing link to Create Private Message endpoint

In the Private Messages section, the end of the paragraph includes a bit of Markdown that looks like it's missing its link:

```
This Channel type differs from others in that it is designed
to provide a simple, combined API for Channel creation, 
euse and Message sending. You can only create 
net.app.core.pm Channels via the [create private message Channel endpoint].
```

Looks like that link should point to:
http://developers.app.net/docs/resources/message/lifecycle/#create-a-message