Update the image link in the doc.
Update the image link in the doc.

Change-Id: I9c6faa8729cafb9fac85069be1180d3840712095
related-bug: #1645092

eonpatapon/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller