Fix syntax of docker.cleanup.image example
Fix syntax of docker.cleanup.image example

I found this syntax issue because I copy-pasted the snippet into my config file and attempted to run Nomad with it. Whoops
hooklift/nomad,Ashald/nomad,dvusboy/nomad,frundh/nomad,dvusboy/nomad,diptanu/nomad,achanda/nomad,hooklift/nomad,Ashald/nomad,barnardb/nomad,nak3/nomad,hashicorp/nomad,barnardb/nomad,kmalec/nomad,barnardb/nomad,Ashald/nomad,diptanu/nomad,nak3/nomad,frundh/nomad,hooklift/nomad,nak3/nomad,Ashald/nomad,barnardb/nomad,achanda/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,frundh/nomad,Ashald/nomad,kmalec/nomad,dvusboy/nomad,nak3/nomad,Ashald/nomad,frundh/nomad,nak3/nomad,hooklift/nomad,burdandrei/nomad,kmalec/nomad,barnardb/nomad,barnardb/nomad,novilabs/nomad,nak3/nomad,frundh/nomad,kmalec/nomad,hashicorp/nomad,kmalec/nomad,hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,nak3/nomad,novilabs/nomad,burdandrei/nomad,diptanu/nomad,hooklift/nomad,dvusboy/nomad,diptanu/nomad,kmalec/nomad,achanda/nomad,dvusboy/nomad,burdandrei/nomad,kmalec/nomad,burdandrei/nomad,burdandrei/nomad,diptanu/nomad,novilabs/nomad,achanda/nomad,achanda/nomad,barnardb/nomad,hooklift/nomad,novilabs/nomad,achanda/nomad,frundh/nomad,novilabs/nomad,diptanu/nomad,diptanu/nomad,frundh/nomad,Ashald/nomad,hooklift/nomad,novilabs/nomad,hashicorp/nomad,dvusboy/nomad,achanda/nomad,novilabs/nomad