Small change.

Author: dashohoxha

Dockerfile

@@ -1,5 +1,5 @@
 ### This file can be used to build a docker image like this:
-###   docker build --tag='btr_client' .
+###   `docker build --tag=btr_client`
 ### It can also be used for automated builds in Docker Hub
 ### (see https://docs.docker.com/userguide/dockerrepos/#automated-builds)