fix docker tag for Faiss HNSW (#496)

Fixed an error that occurred when executing in an environment where the
docker image for faiss_hnsw existed and the docker image for faiss did
not exist.

Author: koron

ann_benchmarks/algorithms/faiss_hnsw/config.yml

@@ -3,7 +3,7 @@ float:
   - base_args: ['@metric']
     constructor: FaissHNSW
     disabled: false
-    docker_tag: ann-benchmarks-faiss
+    docker_tag: ann-benchmarks-faiss_hnsw
     module: ann_benchmarks.algorithms.faiss_hnsw
     name: hnsw(faiss)
     run_groups: