Fixed algorithm str when using FLAT on redisearch

[filipecosta90]

Error to fix:

got a train set of size (1000000 * 960)
got 1000 queries
running 31 out of them
Running query argument group 1 of 1...
Run 1/1...
qps: 0.5204312185441995
Traceback (most recent call last):
  File "/home/ubuntu/ann-benchmarks/run.py", line 12, in <module>
    main()
  File "/home/ubuntu/ann-benchmarks/ann_benchmarks/main.py", line 316, in main
    run_worker(1, args, queue)
  File "/home/ubuntu/ann-benchmarks/ann_benchmarks/main.py", line 41, in run_worker
    run(definition, args.dataset, args.count, args.runs, args.batch,
  File "/home/ubuntu/ann-benchmarks/ann_benchmarks/runner.py", line 178, in run
    descriptor, results = run_individual_query(
  File "/home/ubuntu/ann-benchmarks/ann_benchmarks/runner.py", line 94, in run_individual_query
    "name": str(algo),
  File "/home/ubuntu/ann-benchmarks/ann_benchmarks/algorithms/redisearch.py", line 90, in __str__
    return self.name + f", efRuntime: {self.ef}"
AttributeError: 'RediSearch' object has no attribute 'ef'