doc/config: mark ssh allowedSigners example as literal

The discussion for gpg.ssh.allowedSignersFile shows an example string
that contains "[email protected],[email protected]". 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.: [email protected][1],[email protected][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. [email protected]
           mailto:[email protected]

        2. [email protected]
           mailto:[email protected]

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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

Documentation/config/gpg.txt

@@ -44,7 +44,7 @@ gpg.ssh.allowedSignersFile::
 	A file containing ssh public keys which you are willing to trust.
 	The file consists of one or more lines of principals followed by an ssh
 	public key.
-	e.g.: [email protected],[email protected] ssh-rsa AAAAX1...
+	e.g.: `[email protected],[email protected] ssh-rsa AAAAX1...`
 	See ssh-keygen(1) "ALLOWED SIGNERS" for details.
 	The principal is only used to identify the key and is available when
 	verifying a signature.