class06: add document link

classes/class06/meta.json

@@ -2,6 +2,6 @@
     "name": "Class 06 - Privilege Escalation and Persistence",
     "id": "class-06",
     "description": "The sixth class focuses on privilege escalation, persistence and side channel attacks. Please open the Google document provided to all registered students and follow the document.",
-    "google_doc_url": "",
+    "google_doc_url": "https://docs.google.com/document/d/15MYxVJOZGK8i1hGte9BGTYUbaOik8KrTnFyH256NINg/edit",
     "yt_recording_url": ""
 }