correct name of docker container

Author: stargaser

README_firefly.md

@@ -16,7 +16,7 @@ Here are Docker commands used to test this notebook:
 ```
 docker run -itd -p 9745:9745 -v `pwd`:/home/vagrant/mnt --name lsst2 lsstsqre/centos:7-stack-lsst_distrib-d_2017_12_14
 
-docker exec -it lsst /bin/bash
+docker exec -it lsst2 /bin/bash
 ```
 
 Then in the shell inside the container: