doc: interpret-trailers: fix example

doc: interpret-trailers: fix example

We need to provide `--trailer sign` since the command won’t output
anything if you don’t give it an input and/or a
`--trailer`. Furthermore, the message which already contains an s-o-b is
wrong:

    $ git interpret-trailers --trailer sign <msg.txt
    Signed-off-by: Alice <alice@example.com>
    Signed-off-by: Alice <alice@example.com>
This can’t be what was originally intended.

So change the messages in this example to use the typical
“subject/message” file.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>