[docs] streaming contents: use app.response_class
[docs] streaming contents: use app.response_class

Examples should use app.response_class to encourage code that doesn't need to be changed should the response class be customized later on.
pallets/flask,mitsuhiko/flask,drewja/flask,pallets/flask,fkazimierczak/flask,pallets/flask,fkazimierczak/flask,fkazimierczak/flask,drewja/flask,mitsuhiko/flask,drewja/flask