Wrong code sample in docs
I'm using peewee for the first time but it seems to me that the docs on [querying#joining-on-multiple-tables](http://peewee.readthedocs.org/en/latest/peewee/querying.html#joining-on-multiple-tables) has two wrong code samples:

``` python
User.join(Tweet).join(Comment)
```

``` python
# Join the Artist table on both `Ablum` and `Genre`.
Artist.join(Album).switch(Artist).join(Genre)
```

Is the `select()` missing?
