Fix broken link in Greenstack readme
Fix broken link in Greenstack readme

Change-Id: Ic08a61e56174a0631f793c2f04b65758f774785a
Reviewed-on: http://review.couchbase.org/59310
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

couchbase/memcached,couchbase/memcached,daverigby/memcached,daverigby/memcached,daverigby/kv_engine,daverigby/kv_engine,daverigby/memcached,couchbase/memcached,owendCB/memcached,owendCB/memcached,daverigby/kv_engine,owendCB/memcached,daverigby/memcached,couchbase/memcached,owendCB/memcached,daverigby/kv_engine