Fix GDTF Spec link (#2563)

Author: peternewman

plugins/gdtf/plugin.json

@@ -15,7 +15,7 @@
     "needed ;)</p>"
   ],
   "links": {
-    "GDTF file description": "https://gdtf-share.com/wiki/GDTF_File_Description",
+    "GDTF file description": "https://github.com/mvrdevelopment/spec/blob/main/gdtf-spec.md",
     "GDTF Share": "https://gdtf-share.com/",
     "Robe GDTF repository": "https://gitlab.com/petrvanek/gdtf-libraries",
     "Problems in GDTF": "https://lightconsole.dev/2020/04/07/gdtf-good-idea-poor-execution/"