Fix typo.

Author: silug

README.md

@@ -3,5 +3,5 @@
 This repo contains a Docker configuration for a basic CentOS 7 + Puppet AIO system
 suitable for demonstration purposes.  To use it, run the following:
 
-    docker pull puppetbootstrap:sandbox
+    docker pull puppetbootstrap/sandbox
     docker exec -it sandbox /bin/bash