Remove `PublisherDetail.get_context_data()` from documentation

[Davit Gachechiladze]

The following code snippet is from ​https://docs.djangoproject.com/en/2.2/topics/class-based-views/mixins/

from django.views.generic import ListView
from django.views.generic.detail import SingleObjectMixin
from books.models import Publisher

class PublisherDetail(SingleObjectMixin, ListView):
    paginate_by = 2
    template_name = "books/publisher_detail.html"

    def get(self, request, *args, **kwargs):
        self.object = self.get_object(queryset=Publisher.objects.all())
        return super().get(request, *args, **kwargs)

    def get_context_data(self, **kwargs):
        context = super().get_context_data(**kwargs)
        context['publisher'] = self.object
        return context

    def get_queryset(self):
        return self.object.book_set.all()

PublisherDetail.get_context_data(self, **kwargs) is not neccessary here at all, isn't it ? SingleObjectMixin.get_context_data(self, **kwargs) is responsible to add publisher key in context dict, which is called from BaseListView.get(self, request, *args, **kwargs).

​https://github.com/django/django/pull/11658