ax(mining): replace prose Config comment with usage example

AX principle 2: comments must show usage with realistic values, not
restate what the type signature already says.

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

pkg/mining/mining.go

@@ -77,7 +77,7 @@ type SystemInfo struct {
 	InstalledMinersInfo []*InstallationDetails `json:"installed_miners_info"`
 }
 
-// Config represents the configuration for a miner.
+// Config{Miner: "xmrig", Pool: "stratum+tcp://pool.example.com:3333", Wallet: "4ABC...", Threads: 4}
 type Config struct {
 	Miner             string `json:"miner"`
 	Pool              string `json:"pool"`