Diary app: link, credits and better wording

This fixes a link typo to the diary app tutorial, gives credit where due and attempts to explain better what `ClassName#method_name` notation means.

I’m in favour of keeping the `ClassName#method_name` notation in, as it is used extensively in the Ruby world (to mean ‘the `method_name` method in the `ClassName` class). I hope this additional wording is enough to explain where to look in the app, but I’ll be super happy to reword it (or any other parts) again if I can make it clearer.

Thanks to @FloorD for catching the dense lingo. ♥