doc/config: mark ssh allowedSigners example as literal

doc/config: mark ssh allowedSigners example as literal

The discussion for gpg.ssh.allowedSignersFile shows an example string
that contains "user1@example.com,user2@example.com". Asciidoc thinks
these are real email addresses and generates "mailto" footnotes for
them. This makes the rendered content more confusing, as it has extra
"[1]" markers:

  The file consists of one or more lines of principals followed by an
  ssh public key. e.g.: user1@example.com[1],user2@example.com[2]
  ssh-rsa AAAAX1... See ssh-keygen(1) "ALLOWED SIGNERS" for details.

and also generates pointless notes at the end of the page:

  NOTES
        1. user1@example.com
           mailto:user1@example.com

        2. user2@example.com
           mailto:user2@example.com

We can fix this by putting the example into a backtick literal block.
That inhibits the mailto generation, and as a bonus typesets the example
text in a way that sets it off from the regular prose (a tt font for
html, or bold in the roff manpage).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>