crypto/ssh: minor comment change (trivial)

Fixed a function comment.

Change-Id: I1185c0c849abbb8244c6282fa6036fa6939c3309
Reviewed-on: https://go-review.googlesource.com/23971
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/agent/client.go

@@ -536,7 +536,7 @@ type ed25519CertMsg struct {
 	Constraints []byte `ssh:"rest"`
 }
 
-// Insert adds a private key to the agent. If a certificate is given,
+// Add adds a private key to the agent. If a certificate is given,
 // that certificate is added instead as public key.
 func (c *client) Add(key AddedKey) error {
 	var constraints []byte