specify prefetch spec in nav-speculation (#1007)

Author: srwei

index.bs

@@ -14131,6 +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://wicg.github.io/nav-speculation/prefetch.html">nav-speculation</a>
 
 1. <a href="https://webbluetoothcg.github.io/web-bluetooth/">Web Bluetooth</a>