Update website/docs/api/router.md

Co-authored-by: Jintao Zhang <[email protected]>

Author: samzong

website/docs/api/router.md

@@ -253,7 +253,7 @@ sum by (model) (increase(llm_model_cost_total{currency="USD"}[1h]))
 sum by (reason_code) (increase(llm_routing_reason_codes_total[15m]))
 ```
 
-### Error Metrics
+### Request Error Metrics
 
 The router tracks request errors categorized by failure reason for monitoring and debugging.