Fix the setup of the cookbook example

`ingredients` and `recipes` from [examples/cookbook](https://github.com/graphql-python/graphene-django/tree/master/examples/cookbook) are included in cookbook/cookbook. 
By following the tutorial `ingredients` and `recipes` are in `cookbook/`.

Two possibilities : 
1) There is a mistake in the doc and I can correct it by adding a `cd cookbook` (as done by the PR)
2) Correct the [examples/cookbook](https://github.com/graphql-python/graphene-django/tree/master/examples/cookbook).

Solution 1 is easier but  I prefer the solution 2 because it is closer of what is done in the [django tutorial](https://docs.djangoproject.com/en/1.10/intro/tutorial01/).

docs/tutorial.rst

@@ -31,6 +31,7 @@ We will setup the project, create the following:
 
     # Set up a new project with a single application
     django-admin.py startproject cookbook .  # Note the trailing '.' character
+    cd cookbook
     django-admin.py startapp ingredients
 
 Now sync your database for the first time: