aws/transcriber: fix unsynced_translate_src_%u presence

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1930>

docs/plugins/gst_plugins_cache.json

@@ -1183,7 +1183,7 @@
                     "unsynced_translate_src_%%u": {
                         "caps": "application/x-json:\n",
                         "direction": "src",
-                        "presence": "request"
+                        "presence": "sometimes"
                     }
                 },
                 "properties": {

net/aws/src/transcriber/imp.rs

@@ -1007,7 +1007,7 @@ impl ElementImpl for Transcriber {
             let unsynced_sometimes_src_pad_template = gst::PadTemplate::new(
                 "unsynced_translate_src_%u",
                 gst::PadDirection::Src,
-                gst::PadPresence::Request,
+                gst::PadPresence::Sometimes,
                 &src_caps,
             )
             .unwrap();