salt.auth.django docs use wrong model name

[mattLLVW]

salt.auth.django doc uses:

class SaltExternalAuthModel(models.Model):
    user_fk = models.ForeignKey(auth.User)
    minion_matcher = models.CharField()
    minion_fn = models.CharField()

as an exemple of a django model to auth users.

There's few problems:

• the actual module tries to to fetch minion_or_fn_matcher user attribute.

• CharField should have mandatory max_length kwarg.

• user foreign key should be:

  from django.contrib.auth.models import User
  ...
  user_fk = models.ForeignKey(User, on_delete=models.CASCADE)

Docs should be updated to:

from django.contrib.auth.models import User
class SaltExternalAuthModel(models.Model):
    user_fk = models.ForeignKey(User, on_delete=models.CASCADE)
    minion_or_fn_matcher = models.CharField(max_length=255)
    minion_fn = models.CharField(max_length=255)

[Ch3LL]

would you mind submitting a PR for that change? And thanks for opening the issue :)