Doc: Two links to graphics in README.md broken

[rjhintz]

Update: actually three graphics links

• Under Three Groups:

  • Broken link 1 should be https://github.com/cncf/demo/blob/master/docs/arch.png
  • Broken link 2 should be https://github.com/cncf/demo/blob/master/docs/k8s-simpler.png

• Under Software-defined networking for minions:

  • Broken link 3 should be https://github.com/cncf/demo/blob/master/docs/sdn.png