tour: [Bad description]

[sedyh]

Context: https://go-tour-ru-ru.appspot.com/methods/23

There is need to be some simple description of rot13, like: Characters before the middle letter M are rotated backwards on 13 characters, and other are rotated forwards on 13 characters.
Also here must be a hint like: use inner Read() method of io.Reader to handle []byte from function.

Maybe a small template would be better:

n, err := r.r.Read(b)
for i := 0; i < n; i++ {
    // rot13 transform
}
return n, err