fix documentation formatting (#187)

https://prometheus-api-client-python.readthedocs.io/en/master/source/prometheus_api_client.html#prometheus_api_client.metric.Metric

Was not rendering properly.

Author: rptaylor

prometheus_api_client/metric.py

@@ -39,7 +39,7 @@ class Metric:
         ``Here metric_data is a list of metrics received from prometheus``
 
         ``# only for the first item in the list``
-        ``my_metric_object = Metric(metric_data[0], datetime.timedelta(days=10)) ``
+        ``my_metric_object = Metric(metric_data[0], datetime.timedelta(days=10))``
 
     """