Sync up `examples/basic_auth` with the README

As noted in:
https://github.com/admiralobvious/flask-simpleldap/issues/5
https://github.com/admiralobvious/flask-simpleldap/issues/43

- @login_required can't be used without like that, so it makes sense to
  replace it with the more basic `@basic_auth_required`.
- It's easy to forget to configure `LDAP_HOST`, and many people probably
  are trying to connect to a pre-existing directory, instead of bringing
  up their own.