Update README.md

Upon installation of the tool, the command executed writes rpi-update to /usr/bin detailing a 301 error, indicating the source of the update has changed to https://raw.githubusercontent.com/Hexxeh/rpi-update/master/rpi-update. This address change has been reflected in the proposed file change.
Merge pull request #144 from kcl-lescalan/patch-1

Update README.md