Clarify joi validation exercise

I found the joi validation exercise quite difficult to follow, so i reworked it a little bit.
I encountered following problems:

* The text has many spelling errors (Hopfully i didn't introduce new ones :flushed: :smile:).
* There was no hint that the route has to reply with "login successful" (You have to verify the program to find that out).
* The JS example includes a semicolon which throws a JS error.