ssh: fix typo

Change-Id: I560d7f5a62161cd88361a9fe9982d36f8e25e5af
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/447475
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

ssh/messages.go

@@ -68,7 +68,7 @@ type kexInitMsg struct {
 
 // See RFC 4253, section 8.
 
-// Diffie-Helman
+// Diffie-Hellman
 const msgKexDHInit = 30
 
 type kexDHInitMsg struct {