docs - fix broken external link to point to mailer README instead (#6897)
docs - fix broken external link to point to mailer README instead (#6897)


thisisshi/cloud-custodian,thisisshi/cloud-custodian,alfredgamulo/cloud-custodian,thisisshi/cloud-custodian,alfredgamulo/cloud-custodian,alfredgamulo/cloud-custodian