CCM-12616: Update Mock Mesh doc

Author: simonlabarere

utils/py-mock-mesh/README.md

@@ -21,12 +21,12 @@ This library does the following:
 
    ```bash
    mesh_message_id=$(uuidgen)
-   aws s3 cp <input_file.csv> \
+   aws s3 cp <input_file.json> \
      s3://nhs-<account_id>-eu-west-2-<environment>-dl-non-pii-data/mock-mesh/<mailbox-id>/in/$mesh_message_id \
      --metadata "{\"subject\":\"<SUBJECT>\",\"sender\":\"<SENDER_MAILBOX_ID>\",\"workflow_id\":\"<WORKFLOW_ID>\",\"local_id\":\"<LOCAL_ID>\"}"
    ```
 
-   **Note:** The input file must be a CSV (comma-delimited). Ensure the sender mailbox ID exists in SSM Parameter Store at `/dl/<environment>/senders/<SENDER_MAILBOX_ID>` with valid sender configuration.
+   **Note:** The input file must contain a FHIR resource formatted for the PDM API. Ensure the sender mailbox ID exists in SSM Parameter Store at `/dl/<environment>/senders/<SENDER_ID>` with valid sender configuration.
 
 2. **Trigger the MESH poll lambda** by pressing 'Test' in the AWS console, or wait for the scheduled poll (every 5 minutes).