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

AX-2 violation: comment restated the type name rather than showing a
concrete usage example. Replaced with a call site example showing how
to call GetStats and access the Hashrate field.

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

pkg/mining/mining.go

@@ -243,7 +243,8 @@ func isValidAlgo(algo string) bool {
 	return true
 }
 
-// PerformanceMetrics represents the performance metrics for a miner.
+// metrics, err := miner.GetStats(ctx)
+// if metrics.Hashrate > 0 { log.Printf("%.2f H/s", float64(metrics.Hashrate)) }
 type PerformanceMetrics struct {
 	Hashrate      int                    `json:"hashrate"`
 	Shares        int                    `json:"shares"`