Bump django from 2.1.6 to 2.1.9 in /examples/cookbook-plain

Bumps [django](https://github.com/django/django) from 2.1.6 to 2.1.9.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.1.6...2.1.9)

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

Author: dependabot[bot]

examples/cookbook-plain/requirements.txt

@@ -1,4 +1,4 @@
 graphene
 graphene-django
 graphql-core>=2.1rc1
-django==2.1.6
+django==2.1.9