webrtcsink: fix naming of error dot files for discovery pipelines

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1843>

net/webrtc/src/webrtcsink/imp.rs

@@ -4018,7 +4018,7 @@ impl BaseWebRTCSink {
                             gst::warning!(CAT, imp = self, "Error in discovery pipeline: {err:#?}");
                             pipe.0.debug_to_dot_file_with_ts(
                                 gst::DebugGraphDetails::all(),
-                                "webrtcsink-discovery-error",
+                                format!("webrtcsink-discovery-{}-error", pipe.0.name()),
                             );
                             break Err(err.error().into());
                         }