Clarify restricted directory name character list
Clarify restricted directory name character list

Per suggestion from @inikulshin
in 
https://github.com/rabbitmq/rabbitmq-website/issues/786#issuecomment-502581233.

rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website,rabbitmq/rabbitmq-website