Spelling fix for smtp-mail-queue-dir

* mu4e/mu4e-main.el (mu4e~main-toggle-mail-sending-mode): correct
  error message to say `smtpmail-queue-dir' instead of
  `smtp-queue-dir'

Signed-off-by: jmickey <jmickey@netapp.com>
Merge pull request #137 from codemac/smtp

* mu4e: spelling fix for smtp-mail-queue-dir