Fix help command replaces prefix twice

Hi,

I fixed `hubot help` command replaces prefix twice if the Robot has custom name.

In case `HUBOT_NAME=MyHubot`:

```
MyMyHubot help - Displays all of the help commands that MyHubot knows about.
```

I fixed it to

```
MyHubot help - Displays all of the help commands that MyHubot knows about.
```