rtp: Take the parameter from the arguments

$ gst rtp_replay.st -a FILE
diff --git a/openbsc/contrib/rtp/rtp_replay.st b/openbsc/contrib/rtp/rtp_replay.st
index a281570..7fa9a4c 100644
--- a/openbsc/contrib/rtp/rtp_replay.st
+++ b/openbsc/contrib/rtp/rtp_replay.st
@@ -10,7 +10,7 @@
     | replay |
 
 
-    replay := RTPReplay on: 'rtp_ssrc6976010.240.240.1_to_10.240.240.50.state'.
+    replay := RTPReplay on: Smalltalk arguments first.
 
     Transcript nextPutAll: 'Going to stream now'; nl.
     replay streamAudio: '127.0.0.1' port: 4000.