Bump django from 3.0 to 3.0.3 in /examples/cookbook-plain (#877)

Bumps [django](https://github.com/django/django) from 3.0 to 3.0.3.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@3.0...3.0.3)

Signed-off-by: dependabot[bot] <support@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==3.0
+django==3.0.3