chore: correct docstring for log_metrics

Author: Tom McCarthy

Diff for: aws_lambda_powertools/metrics/metrics.py

@@ -86,7 +86,7 @@ def log_metrics(self, lambda_handler: Callable[[Any, Any], Any] = None):
         -------
         **Lambda function using tracer and metrics decorators**
 
-            metrics = Metrics()
+            metrics = Metrics(service="payment")
             tracer = Tracer(service="payment")
 
             @tracer.capture_lambda_handler