Add further and clear instruction to see effect of Http404 in tutorial 3

[Samuel Hartmann]

In third part of the tutorial we have the description for ​Raising a 404 error.
A further instruction on how to display the visual result of the Http404 error in the browser would be helpful.

Therefore I've already added a way to describe shortly and clearly in the ​pull request #14644.

[Mariusz Felisiak]

Thanks for this proposition, however I don't think that any further clarification is required. It's not important at this point, what the 404 error page looks like. We try to avoid adding any new materials to tutorials, unless it's a big problem.