Fix a typo in filexfer example (fixes: #1083)

The example "worked" due to a global variable, but this was not the
desired code.

Thanks to @angererc for spotting this!

examples/datachannel-filexfer/filexfer.py

@@ -31,7 +31,7 @@ async def consume_signaling(pc, signaling):
  break
 
 
-async def run_answer(pc, signaling, filename):
+async def run_answer(pc, signaling, fp):
  await signaling.connect()
 
  @pc.on("datachannel")