R1:b645af981bbe

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