Fix ingredient model (#1258)

Add the required positional argument: 'on_delete'

docs/tutorial-relay.rst

@@ -70,7 +70,7 @@ Let's get started with these models:
     class Ingredient(models.Model):
         name = models.CharField(max_length=100)
         notes = models.TextField()
-        category = models.ForeignKey(Category, related_name='ingredients')
+        category = models.ForeignKey(Category, related_name='ingredients', on_delete=models.CASCADE)
 
         def __str__(self):
             return self.name