cmd/tailscale/cli/web: fix typo where the html template data was being

replaced instead of being appended to.

Signed-off-by: Maisem Ali <maisem@tailscale.com>
(cherry picked from commit 9e8a432146)

cmd/tailscale/cli/web.go

@@ -375,7 +375,7 @@ func webHandler(w http.ResponseWriter, r *http.Request) {
 			data.AdvertiseExitNode = true
 		} else {
 			if data.AdvertiseRoutes != "" {
-				data.AdvertiseRoutes = ","
+				data.AdvertiseRoutes += ","
 			}
 			data.AdvertiseRoutes += r.String()
 		}