Fix Sheriff-o-matic README to use dev_appserver.py

It no longer works to use Python's SimpelHTTPServer.

NOTRY=true
TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/616733002

git-svn-id: svn://svn.chromium.org/blink/trunk@182877 bbb929c8-8fbe-4397-9dbb-9b2b20218538