ax(mining): replace prose comment with usage example on findAvailablePort

AX Principle 2 — comments must show HOW with real values, not restate
what the function name already says.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/manager.go

@@ -213,7 +213,9 @@ func (m *Manager) autostartMiners() {
 	}
 }
 
-// findAvailablePort finds an available TCP port on the local machine.
+// port, err := findAvailablePort()
+// if err != nil { return 0, err }
+// config.HTTPPort = port
 func findAvailablePort() (int, error) {
 	addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
 	if err != nil {