Docs: Update boot2docker shellinit example to use 'eval'

The boot2docker documentation has since changed the recommended way to
use shellinit - see boot2docker/boot2docker#786.

Signed-off-by: Ed Morley <[email protected]>

docs/cli.md

@@ -157,7 +157,7 @@ By default, if there are existing containers for a service, `docker-compose up`
 Several environment variables are available for you to configure Compose's behaviour.
 
 Variables starting with `DOCKER_` are the same as those used to configure the
-Docker command-line client. If you're using boot2docker, `$(boot2docker shellinit)`
+Docker command-line client. If you're using boot2docker, `eval "$(boot2docker shellinit)"`
 will set them to their correct values.
 
 ### COMPOSE\_PROJECT\_NAME