Fix Fontmatter (#9520)

https://github.com/FreshRSS/FreshRSS/pull/9509 broke CI

docs/threat_model.md

@@ -1,3 +1,7 @@
+---
+lang: en
+---
+
 # FreshRSS threat model
 
 FreshRSS is a self-hosted RSS/Atom reader.