docs: add missing import to aws-lambda (open-telemetry#3154)

Author: Kludex

instrumentation/opentelemetry-instrumentation-aws-lambda/src/opentelemetry/instrumentation/aws_lambda/__init__.py

@@ -55,6 +55,7 @@ def lambda_handler(event, context):
 .. code:: python
 
     from opentelemetry.instrumentation.aws_lambda import AwsLambdaInstrumentor
+    from opentelemetry.propagate import get_global_textmap
 
     def custom_event_context_extractor(lambda_event):
         # If the `TraceContextTextMapPropagator` is the global propagator, we