ssh/agent: update documentation link
Fixes golang/go#24982
Run-TryBot: Han-Wen Nienhuys
TryBot-Result: Gobot Gobot
// ssh-agent process using the sample server.
//
// References:
-// [PROTOCOL.agent]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.agent?rev=HEAD
+// [PROTOCOL.agent]: https://tools.ietf.org/html/draft-miller-ssh-agent-00
package agent // import "golang.org/x/crypto/ssh/agent"
import (
