chg: [ransomware] academic reference added

ransomware/machinetag.json

@@ -2,11 +2,12 @@
   "namespace": "ransomware",
   "expanded": "ransomware types and elements",
   "description": "Ransomware is used to define ransomware types and the elements that compose them.",
-  "version": 5,
+  "version": 6,
   "refs": [
     "https://www.symantec.com/content/en/us/enterprise/media/security_response/whitepapers/the-evolution-of-ransomware.pdf",
     "https://docs.apwg.org/ecrimeresearch/2018/5357083.pdf",
-    "https://bartblaze.blogspot.com/p/the-purpose-of-ransomware.html"
+    "https://bartblaze.blogspot.com/p/the-purpose-of-ransomware.html",
+    "https://arxiv.org/pdf/2102.06249.pdf"
   ],
   "predicates": [
     {