Fix incorrect description of yarn.timeline-service.enabled in tez-site.xml

Update description of yarn.timeline-service.enabled in tez-site.xml of BIGTOP stack - 3.2.0 to  "Indicate to clients whether timeline service is enabled or not. If enabled, clients will put entities and events to the timeline server." 

When we hover on the text-box of the above config on ambari-server TEZ configs misleading description is shown.