Bump django from 1.11.20 to 1.11.21 in /examples/cookbook (#670)

Bumps [django](https://github.com/django/django) from 1.11.20 to 1.11.21.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@1.11.20...1.11.21)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

examples/cookbook/requirements.txt

@@ -1,5 +1,5 @@
 graphene
 graphene-django
 graphql-core>=2.1rc1
-django==1.11.20
+django==1.11.21
 django-filter>=2