Bump django from 3.1.6 to 3.1.8 in /examples/cookbook-plain (#1157)

Bumps [django](https://github.com/django/django) from 3.1.6 to 3.1.8.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@3.1.6...3.1.8)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

examples/cookbook-plain/requirements.txt

@@ -1,4 +1,4 @@
 graphene>=2.1,<3
 graphene-django>=2.1,<3
 graphql-core>=2.1,<3
-django==3.1.6
+django==3.1.8