Fix incorrect and missing i18n text domains

/includes/admin/ajax-actions.php
* [L28]  __  Incorrect text domain used ("rcp")

/includes/admin/referrals/referrals.php
* [L410]  __  Missing text domain
* [L422]  __  Missing text domain

/includes/admin/visits/visits.php
* [L219]  __  Missing text domain