Fix misplaced comma in TSDB track

[gbanasiak]

#743 introduced this problem:

% esrally info --track=tsdb --track-params="index_mode:standard"

    ____        ____
   / __ \____ _/ / /_  __
  / /_/ / __ `/ / / / / /
 / _, _/ /_/ / / / /_/ /
/_/ |_|\__,_/_/_/\__, /
                /____/

[ERROR] Cannot info. Could not load '/Users/grzegorz/.rally/benchmarks/tracks/default/tsdb/track.json': Expecting ',' delimiter: line 510 column 9 (char 10509). Lines containing the error:

          "iterations": 100
        }
        
        {
--------^ Error is here
          "operation": "esql-fetch-500",
          "warmup-iterations": 50,

The complete track has been written to '/var/folders/fy/mlp_1k2s5yx_vm9gt82dc3140000gn/T/tmpso8ke_8q.json' for diagnosis.

[gareth-ellis]

LGTM