Corrected label for vb_active_queue_drain

Change-Id: I1ba7cef76e526961efb76d008663eee9267cdb74
Reviewed-on: http://review.couchbase.org/13054
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>

src/menelaus_stats.erl

@@ -831,7 +831,7 @@ membase_stats_description() ->
                          {desc,<<"Total number of items per second being put on the disk queue in this bucket (measured from ep_diskqueue_fill)">>}]},
                 {struct,[{title,<<"drain rate">>},
                          {name,<<"vb_active_queue_drain">>},
-                         {desc,<<"Number of active items per second being written to disk in this bucket (measured from vb_pending_queue_drain)">>}]},
+                         {desc,<<"Number of active items per second being written to disk in this bucket (measured from vb_active_queue_drain)">>}]},
                 {struct,[{title,<<"drain rate">>},
                          {name,<<"vb_replica_queue_drain">>},
                          {desc,<<"Number of replica items per second being written to disk in this bucket (measured from vb_replica_queue_drain)">>}]},