Bump django from 2.2.4 to 2.2.8 in /examples/cookbook-plain (#822)

Bumps [django](https://github.com/django/django) from 2.2.4 to 2.2.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.4...2.2.8)

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

Author: dependabot[bot]

examples/cookbook-plain/requirements.txt

@@ -1,4 +1,4 @@
 graphene
 graphene-django
 graphql-core>=2.1rc1
-django==2.2.4
+django==2.2.8