add external speculation spec ref (#1003)

Author: srwei

index.bs

@@ -14131,4 +14131,6 @@ The following external specifications define additional WebDriver BiDi modules:
 
 1. <a href="https://www.w3.org/TR/permissions/">Permissions</a>
 
+1. <a href="https://wicg.github.io/nav-speculation/">nav-speculation</a>
+
 1. <a href="https://webbluetoothcg.github.io/web-bluetooth/">Web Bluetooth</a>