srtp: Add "roc" caps field to the gst-launch example

The currrent example was broken since 1.8.3 it seems.

https://bugzilla.gnome.org/show_bug.cgi?id=786304
diff --git a/ext/srtp/gstsrtpdec.c b/ext/srtp/gstsrtpdec.c
index d44086f..67c05d3 100644
--- a/ext/srtp/gstsrtpdec.c
+++ b/ext/srtp/gstsrtpdec.c
@@ -98,7 +98,7 @@
  *
  * ## Example pipelines
  * |[
- * gst-launch-1.0 udpsrc port=5004 caps='application/x-srtp, payload=(int)8, ssrc=(uint)1356955624, srtp-key=(buffer)012345678901234567890123456789012345678901234567890123456789, srtp-cipher=(string)aes-128-icm, srtp-auth=(string)hmac-sha1-80, srtcp-cipher=(string)aes-128-icm, srtcp-auth=(string)hmac-sha1-80' !  srtpdec ! rtppcmadepay ! alawdec ! pulsesink
+ * gst-launch-1.0 udpsrc port=5004 caps='application/x-srtp, payload=(int)8, ssrc=(uint)1356955624, srtp-key=(buffer)012345678901234567890123456789012345678901234567890123456789, srtp-cipher=(string)aes-128-icm, srtp-auth=(string)hmac-sha1-80, srtcp-cipher=(string)aes-128-icm, srtcp-auth=(string)hmac-sha1-80, roc=(uint)0' !  srtpdec ! rtppcmadepay ! alawdec ! pulsesink
  * ]| Receive PCMA SRTP packets through UDP using caps to specify
  * master key and protection.
  * |[