Update README.md

Author: tianchu

README.md

@@ -75,6 +75,7 @@ Custom metrics can be submitted using `lambda_metric` and the Lambda handler fun
 **IMPORTANT NOTE:** If you have already been submitting the same custom metric as non-distribution metric (e.g., gauge, count, or histogram) without using the Datadog Lambda Layer, you MUST pick a new metric name to use for `lambda_metric`. Otherwise that existing metric will be converted to a distribution metric and the historical data prior to the conversion will be no longer queryable.
 
 ```python
+from datadog_lambda.wrapper import datadog_lambda_wrapper
 from datadog_lambda.metric import lambda_metric
 
 @datadog_lambda_wrapper