Update telemetry-api-reference.md

add missing header parameter Lambda-Extension-Identifier required for Telemetry Api

Author: M1100

doc_source/telemetry-api-reference.md

@@ -19,6 +19,7 @@ To subscribe to a telemetry stream, a Lambda extension can send a Subscribe API
 + **Method** – `PUT`
 + **Headers**
   + `Content-Type`: `application/json`
+  + `Lambda-Extension-Identifier`: the `Lambda-Extension-Identifier` value received upon registering your extension\.
 + **Request body parameters**
   + **schemaVersion**
     + Required: Yes
@@ -123,4 +124,4 @@ Here are some additional response codes that the extension can receive:
 + 200 – Request completed successfully
 + 202 – Request accepted\. Subscription request response in local testing environment
 + 400 – Bad request
-+ 500 – Service error
++ 500 – Service error