chore(doc): fix code snippet (#493)

Author: krokoko

src/cdk-lib/bedrock/README.md

@@ -386,7 +386,7 @@ actionGroup = bedrock.AgentActionGroup(self,
     action_group_name="query-library",
     description="Use these functions to get information about the books in the library.",
     action_group_executor= bedrock.ActionGroupExecutor(
-      lambda=action_group_function
+      lambda_=action_group_function
     ),
     action_group_state="ENABLED",
     api_schema=bedrock.ApiSchema.from_asset("action-group.yaml"))