Fix lint errors in ldap_user_dn_example parameter

There is no $username variable anywhere in the rabbitmq module.
${username} is not a valid cn. While most likely no one was using
dc=example,dc=com anyway, this patch at least changes the example value
to one that lints.
Merge pull request #283 from cmurphy/ldap

Fix lint errors in ldap_user_dn_example parameter