Correct TypeScript signature of client.sendCompleteEvent (#1997)

* Correct TypeScript signature of client.sendCompleteEvent

* Revert renaming txnId to txnIdOrCallback

src/client.ts

@@ -3405,7 +3405,7 @@ export class MatrixClient extends EventEmitter {
     /**
      * @param {string} roomId
      * @param {object} eventObject An object with the partial structure of an event, to which event_id, user_id, room_id and origin_server_ts will be added.
-     * @param {string} txnId the txnId.
+     * @param {string} txnId Optional.
      * @param {module:client.callback} callback Optional.
      * @return {Promise} Resolves: to an empty object {}
      * @return {module:http-api.MatrixError} Rejects: with an error response.
@@ -3413,7 +3413,7 @@ export class MatrixClient extends EventEmitter {
     private sendCompleteEvent(
         roomId: string,
         eventObject: any,
-        txnId: string,
+        txnId?: string,
         callback?: Callback,
     ): Promise<ISendEventResponse> {
         if (utils.isFunction(txnId)) {