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 <[e-mail]>
Cc: Eric Biggers <[e-mail]>
Cc: Herbert Xu <[e-mail]>
Cc: Mauro Carvalho Chehab <[e-mail]>
Cc: [e-mail]
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>

Documentation/security/siphash.rst

···
 ============================
 
 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.