Update GardeningServer/README to point to the correct URL

The main HTML is now called sheriff-o-matic.html

BUG=none
TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/446763002

git-svn-id: svn://svn.chromium.org/blink/trunk@179566 bbb929c8-8fbe-4397-9dbb-9b2b20218538