Documentation: siphash: enclose HalfSipHash usage example in the literal block

Render usage example of HalfSipHash function as code block by using
literal block syntax.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>