Sorry for the late reply (I only found this after Googling my own project) but as @ByteCommander and @karel stated, the real instructions are on the GitHub repo itself and should be more future-proof, I recommend you check that out instead for the most up-to-date documentation, and if you have issues installing stuff or find bugs, use that bug tracker instead. I reply to all issues.
Why did I get a 404 error?
The reason you get a 404 is due to the fact that 'stegcracker' is now a directory containing Python source code, which you obviously can't download with one http request so GitHub returns a 404 html page instead which bash tried to run as code and failed miserably to.
Why did you change it?
I had to rewrite the entire codebase as the project was initially written as a bash script for myself in the 1.0 version and was terribly slow (full details are in this issue and this blog post). TL;DR, small hobby project got out of hand, it had no threading and every password attempt had to wait for steghide to test if it worked and wait for the terminal to output the current attempt, rewrite was 2960% faster (a lot).
Regarding the school assignment though, what kind of assignment did you get?
Never knew my projects got used in schools, quite cool to see :)