Fixed broken image link in 04-cond.ipynb

I noticed a broken link to an image as I was perusing the novice python material, and here is a quick fix. 

Also, just out of curiosity, I'm a bit confused as to the full `src` string for all of the images in the various notebooks. There isn't an explicit `files` directory, yet all of the markup refers to it (all of the images reside in `./img` relative to the notebooks) . If someone could clarify the syntax in specifying the location of the image files, I'd appreciate it.