fix docker-compose.yml have not link redis (docker#5602)

rt, if web have not link redis, `redis = Redis(host='redis', port=6379)` will can not recognize the host('redis')

Author: x1ah

compose/gettingstarted.md

@@ -87,6 +87,8 @@ the following:
         build: .
         ports:
          - "5000:5000"
+        links:
+         - redis
       redis:
         image: "redis:alpine"