message: typo fix in gpu_cache README
diff: # GPU embedding cache
  This project implements a embedding cache on GPU memory designed for CTR inference workload.
  -The cache will store the hot [embedding id, embedding vectors] pairs on GPU memory, thus can reduce the trafic to parameter server when doing embedding table lookup.
  +The cache will store the hot [embedding id, embedding vectors] pairs on GPU memory, thus can reduce the traffic to parameter server when doing embedding table lookup.
  -The cache is desinged for CTR inference, it has following features/restrictions:
  +The cache is designed for CTR inference, it has following features/restrictions:
  * The cache is read-only: modifying the embedding vector of a embedding id is not supported.
  * All the backup memory side operation is performed by parameter server(prefetching, latency hiding etc.).
  * Single-GPU design, each cache belongs to one GPU.
  