Bump django from 2.2.8 to 3.0 in /examples/cookbook (#825)

Bumps [django](https://github.com/django/django) from 2.2.8 to 3.0.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.8...3.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

examples/cookbook/requirements.txt

@@ -1,5 +1,5 @@
 graphene
 graphene-django
 graphql-core>=2.1rc1
-django==2.2.8
+django==3.0
 django-filter>=2