crypto: doc - remove references to ARC4

arc4 is no longer considered secure, so it shouldn't be used, even as
just an example.  Mention serpent and chacha20 instead.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>