Add missing commas in Prefetch docs

    >>> vegetarian_pizzas = Pizza.objects.filter(vegetarian=True)

    >>> Restaurant.objects.prefetch_related(

    ...     Prefetch('pizzas', to_attr('menu')),

    ...     Prefetch('pizzas', queryset=vegetarian_pizzas to_attr='vegetarian_menu'))

    ...     Prefetch('pizzas', queryset=vegetarian_pizzas, to_attr='vegetarian_menu'))

Lookups created with custom ``to_attr`` can still be traversed as usual by other

lookups:

    >>> vegetarian_pizzas = Pizza.objects.filter(vegetarian=True)

    >>> Restaurant.objects.prefetch_related(

    ...     Prefetch('pizzas', queryset=vegetarian_pizzas to_attr='vegetarian_menu'),

    ...     Prefetch('pizzas', queryset=vegetarian_pizzas, to_attr='vegetarian_menu'),

    ...     'vegetarian_menu__toppings')

Using ``to_attr`` is recommended when filtering down the prefetch result as it is