cmd/tailscaled: fix monitor debug tool's output

Logic was backwards, introduced in earlier monitor refactoring last
week in e3df29d488.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/debug.go

@@ -69,7 +69,7 @@ func runMonitor(ctx context.Context) error {
 		return err
 	}
 	mon.RegisterChangeCallback(func(changed bool, st *interfaces.State) {
-		if changed {
+		if !changed {
 			log.Printf("Link monitor fired; no change")
 			return
 		}