[docs] enhanced the basic example

To protect people from starting their exporter multiple times, one has to
evaluate the error returned by http.ListenAndServe().

prometheus/doc.go

@@ -59,7 +59,7 @@
 //    	// The Handler function provides a default handler to expose metrics
 //    	// via an HTTP server. "/metrics" is the usual endpoint for that.
 //    	http.Handle("/metrics", promhttp.Handler())
-//    	http.ListenAndServe(":8080", nil)
+//      log.Fatal(http.ListenAndServe(":8080", nil))
 //    }
 //
 //