docs: dm-integrity: remove reference to ARC4

ARC4 is no longer considered secure, so it shouldn't be used, even as
just an example.

Signed-off-by: Eric Biggers <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

Documentation/admin-guide/device-mapper/dm-integrity.rst

···
 	Encrypt the journal using given algorithm to make sure that the
 	attacker can't read the journal. You can use a block cipher here
 	(such as "cbc(aes)") or a stream cipher (for example "chacha20",
-	"salsa20", "ctr(aes)" or "ecb(arc4)").
+	"salsa20" or "ctr(aes)").
 
 	The journal contains history of last writes to the block device,
 	an attacker reading the journal could see the last sector nubmers