rtprecv: fix SSRC collision event sent in wrong direction

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2351>

net/rtp/src/rtpbin2/rtprecv.rs

@@ -824,7 +824,7 @@ impl RtpRecv {
             // XXX: Another option is to have us rewrite ssrc's instead of asking upstream to do
             // so.
             for ssrc in ssrc_collision {
-                pad.send_event(
+                pad.push_event(
                     gst::event::CustomUpstream::builder(
                         gst::Structure::builder("GstRTPCollision")
                             .field("ssrc", ssrc)