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>

Author: bagasme

Documentation/security/siphash.rst

@@ -132,10 +132,10 @@ Generating a HalfSipHash key
 ============================
 
 Keys should always be generated from a cryptographically secure source of
-random numbers, either using get_random_bytes or get_random_once:
+random numbers, either using get_random_bytes or get_random_once::
 
-hsiphash_key_t key;
-get_random_bytes(&key, sizeof(key));
+	hsiphash_key_t key;
+	get_random_bytes(&key, sizeof(key));
 
 If you're not deriving your key from here, you're doing it wrong.