cmd/derper: update a link from godoc.org to pkg.go.dev
Save a redirect.
@@ -153,7 +153,7 @@ func main() {
<p>
This is a
<a href="https://tailscale.com/">Tailscale</a>
- <a href="https://godoc.org/tailscale.com/derp">DERP</a>
+ <a href="https://pkg.go.dev/tailscale.com/derp">DERP</a>
server.
</p>
`)
