Update list-ssh-keys helptext
Update list-ssh-keys helptext

Refs https://bugs.launchpad.net/juju-core/+bug/1554705

kat-co/juju,bogdanteleaga/juju,marcmolla/juju,anastasiamac/juju,howbazaar/juju,perrito666/juju,perrito666/juju,gabriel-samfira/juju,anastasiamac/juju,mjs/juju,alesstimec/juju,kat-co/juju,makyo/juju,dimitern/juju,kat-co/juju,reedobrien/juju,gabriel-samfira/juju,axw/juju,anastasiamac/juju,frankban/juju,bogdanteleaga/juju,ericsnowcurrently/juju,reedobrien/juju,axw/juju,gabriel-samfira/juju,alesstimec/juju,waigani/juju,AdamIsrael/juju,frankban/juju,perrito666/juju,dimitern/juju,kat-co/juju,kat-co/juju,marcmolla/juju,alesstimec/juju,howbazaar/juju,howbazaar/juju,bac/juju,dooferlad/juju,bz2/juju,reedobrien/juju,waigani/juju,dooferlad/juju,bac/juju,mikemccracken/juju,dooferlad/juju,ericsnowcurrently/juju,mikemccracken/juju,axw/juju,dooferlad/juju,dimitern/juju,reedobrien/juju,gabriel-samfira/juju,alesstimec/juju,dimitern/juju,marcmolla/juju,AdamIsrael/juju,makyo/juju,frankban/juju,anastasiamac/juju,mjs/juju,macgreagoir/juju,dooferlad/juju,bz2/juju,perrito666/juju,axw/juju,bogdanteleaga/juju,bz2/juju,marcmolla/juju,marcmolla/juju,macgreagoir/juju,mjs/juju,mjs/juju,bz2/juju,howbazaar/juju,macgreagoir/juju,anastasiamac/juju,ericsnowcurrently/juju,mikemccracken/juju,bac/juju,mjs/juju,bogdanteleaga/juju,makyo/juju,bz2/juju,dimitern/juju,waigani/juju,mjs/juju,mikemccracken/juju,bac/juju,macgreagoir/juju,makyo/juju,bac/juju,waigani/juju,perrito666/juju,AdamIsrael/juju,frankban/juju,mjs/juju,gabriel-samfira/juju,macgreagoir/juju,axw/juju,mikemccracken/juju,howbazaar/juju,reedobrien/juju,alesstimec/juju,ericsnowcurrently/juju,frankban/juju,ericsnowcurrently/juju,AdamIsrael/juju