Upgrade mention of MIDDLEWARE_CLASSES to newer MIDDLEWARE setting in Django docs

[izabelakowal]

I'm aware that the pyinstrument middleware is using django.utils.deprecation.MiddlewareMixin therefore is compatible with both MIDDLEWARE and the old MIDDLEWARE_CLASSES. However, since the MIDDLEWARE_CLASSES setting was officially deprecated since Django 1.10, this being mentioned in the docs might be a bit confusing to the user.

https://docs.djangoproject.com/en/1.11/ref/settings/#middleware-classes