maybe it's the colon?

_projects/dqn_wifi-rate-adaptation_orca.md

@@ -1,7 +1,7 @@
 ---
 name: Deep Q Network-based Rate Adaptation for IEEE 802.11ac Networks
 desc: Implementation and performance evaluation of reinforcement learning algorithm based on the Deep Q Network approach for MCS rate adaptation in real WiFi networks
-collaborating_projects:
+collaborating_projects
   - openwrt.org
 developers_involved: []
 difficulty: tough