Fix readme file links

[gsfk]

At several points in the READMEs you're instructed to modify files. Sometimes you're given a local link to a file, but often you get a github url that opens in the browser, forcing you to figure out where the file is and navigate the code by hand. I've corrected these to local file links where it made sense.

Also:

• fixed a README image link to point to the image in the repo instead of github (othwise not much sense having the image in the repo... )
• removed one typo

[costero-e]

Very useful fixes for the instructions. Thank you @gsfk!