docs: fix import statement for EventEnvelope (acryldata#97)

docs/guides/developing-a-transformer.md

@@ -23,7 +23,7 @@ print the configuration that is provided when it is created, and print any Event
 ```python
 # custom_transformer.py
 from datahub_actions.transform.transformer import Transformer
-from datahub_actions.event.event import EventEnvelope
+from datahub_actions.event.event_envelope import EventEnvelope
 from datahub_actions.pipeline.pipeline_context import PipelineContext
 from typing import Optional
 
@@ -130,4 +130,4 @@ it without defining the full module path.
 Prerequisites to consideration for inclusion in the core Transformer library include
 
 - **Testing** Define unit tests for your Transformer
-- **Deduplication** Confirm that no existing Transformer serves the same purpose, or can be easily extended to serve the same purpose
+- **Deduplication** Confirm that no existing Transformer serves the same purpose, or can be easily extended to serve the same purpose