Small fix regarding copying in lecture 1 images.jl

[sebastiantk]

In lecture 1.1 the color of some pixels of a copy of philip's head is changed to red. There it is written that this also affects the original image and not only the copy. This is from what I understand incorrect.

I included a suggested change but I am still relatively new to Julia so naturally please change this in any way you see fit for the course. I also did not find any contribution guidelines to the course so I hope this is welcome!

As a final note: thank you for making this great course open to the public! :)