api docs: Fix typo in typing notifications docs.

notification_to should be just to here.

This resulted from the person doing the port into these docs not
understanding the full REQ API.

zerver/openapi/zulip.yaml

@@ -1923,7 +1923,7 @@ paths:
           - stop
         example: start
         required: true
-      - name: notification_to
+      - name: to
         in: query
         description: The recipients of the message being typed, in the same format used by the
           send_message API.  Typing notifications are only supported for private messages,