Change the signature start string to "-- \n"

Change the signature start string from "--\n" to "-- \n" in the cover
mail template, as recommended in Mutt's manual:

    It is *strongly* recommended that you not unset this variable
    [from its default value of "-- \n"] unless your "signature"
    contains just your name. The reason for this is because many
    software packages use "-- \n" to detect your signature. For
    example, Mutt has the ability to highlight the signature in a
    different color in the builtin pager.

Signed-off-by: Karl Hasselström <kha@treskal.com>