fix typo in w05 classes & incorrect contribution URL in how to contribute

[sakerhetspolisen]

I corrected a simple grammatical error: Ett objekt som skapats med en klassen ... became Ett objekt som skapats med klassen.

[sakerhetspolisen]

I also changed the URL that used to link to the How to contribute section of this repo: https://github.com/lunduniversity/introprog#how-to-contribute-to-this-repo became https://github.com/lunduniversity/introprog#how-to-contribute

[bjornregnell]

Thanks for spotting och fixing this!