Fix table definition in metrics doc source
Fix table definition in metrics doc source

patch by Yifan Cai; reviewed by Benjamin Lerer for CASSANDRA-16410

belliottsmith/cassandra,beobal/cassandra,belliottsmith/cassandra,adelapena/cassandra,instaclustr/cassandra,pauloricardomg/cassandra,pauloricardomg/cassandra,szhou1234/cassandra,szhou1234/cassandra,mike-tr-adamson/cassandra,driftx/cassandra,driftx/cassandra,adelapena/cassandra,bdeggleston/cassandra,tjake/cassandra,thelastpickle/cassandra,michaelsembwever/cassandra,driftx/cassandra,apache/cassandra,jrwest/cassandra,michaelsembwever/cassandra,adelapena/cassandra,tjake/cassandra,apache/cassandra,apache/cassandra,jrwest/cassandra,thelastpickle/cassandra,instaclustr/cassandra,blambov/cassandra,tjake/cassandra,jrwest/cassandra,belliottsmith/cassandra,mshuler/cassandra,bdeggleston/cassandra,belliottsmith/cassandra,michaelsembwever/cassandra,blambov/cassandra,thelastpickle/cassandra,instaclustr/cassandra,mshuler/cassandra,szhou1234/cassandra,driftx/cassandra,beobal/cassandra,michaelsembwever/cassandra,szhou1234/cassandra,bdeggleston/cassandra,tjake/cassandra,mshuler/cassandra,pauloricardomg/cassandra,jrwest/cassandra,blambov/cassandra,apache/cassandra,mike-tr-adamson/cassandra,bdeggleston/cassandra,blambov/cassandra,beobal/cassandra,mshuler/cassandra,instaclustr/cassandra,adelapena/cassandra,beobal/cassandra,mike-tr-adamson/cassandra,thelastpickle/cassandra,pauloricardomg/cassandra,mike-tr-adamson/cassandra