Fix broken django-registration, add note, and fix a typo.

Following the instructions at https://github.com/django/djangoproject.com/blob/master/README.txt does not currently work, as the referenced BitBucket fork of `django-registration` is not available anymore. This switches it for another working fork, making the whole thing build again.

Running `./manage.py syncdb --docs` says:

```
Not synced (use migrations):
 - docs
```

So I added a `./manage.py migrate --docs` to the set of instructions, just as in the `syncdb`/`migrate` command right above this step.

Also, fixed a typo in the said README.