Fixed a few typos

Ahh yeah, thanks for that. I actually already had the source in my sources.list so I just faked that step without testing it. I'll probably go with 

```
sudo sh -c 'echo "deb http://download.mono-project.com/repo/debian wheezy main" > /etc/apt/sources.list.d/mono-xamarin.list'
```

As it is a bit more straight forward, but thanks a ton for pointing this out.