Fix the response format of send_knock

This aligns with the current v2 federation endpoints. However, we're still using v1
as a prefix here as it is still the first version of this endpoint.

proposals/2403-knock.md

@@ -249,10 +249,7 @@ Content-Type: application/json
 The event was successfully accepted into the graph by the receiving
 homeserver.
 ```json
-[
-  200,
-  {}
-]
+{}
 ```
 
 # Potential issues