fix start.sh to work on macos (#5019)

remove extra leading / in volume mount command

core/standalone/start.sh

@@ -20,7 +20,7 @@ shift
 docker run --rm -d \
  -h openwhisk --name openwhisk \
  -p 3233:3233 -p 3232:3232 \
- -v //var/run/docker.sock:/var/run/docker.sock \
+ -v /var/run/docker.sock:/var/run/docker.sock \
  "$IMAGE" "$@"
 docker exec openwhisk waitready
 case "$(uname)" in