Add a miss link about apdex documentation. (#4543)

docs/en/setup/backend/backend-setup.md

@@ -102,6 +102,7 @@ system.
 or 3rd party configuration management system.
 1. [Uninstrumented Gateways](uninstrumented-gateways.md). Configure gateways/proxies that are not supported by SkyWalking agent plugins,
 to reflect the delegation in topology graph.
+1. [Apdex threshold](apdex-threshold.md). Configure the thresholds for different services if Apdex calculation is activated in the OAL.
 
 ## Telemetry for backend
 OAP backend cluster itself underlying is a distributed streaming process system. For helping the Ops team,