ssh/agent: update documentation link

Fixes golang/go#24982

Change-Id: I81c646b92230bdcbc7f08de535dd657e86bb03ce
Reviewed-on: https://go-review.googlesource.com/108775
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

@@ -8,7 +8,7 @@
 // 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 (