Correct a missing parameter in "Outbound Mail" documentation

Summary: See https://discourse.phabricator-community.org/t/documentation-error/2079. This command is missing the configuration key.

Test Plan: Ran bin/config set --stdin with no other arguments, got an error about missing key. Ran with ... cluster.mailers, got read from stdin.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19791