rav1enc: document bitrate property unit

See:

e34e772e47/src/rate.rs (L365)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1668>

docs/plugins/gst_plugins_cache.json

@@ -4007,7 +4007,7 @@
                 },
                 "properties": {
                     "bitrate": {
-                        "blurb": "Bitrate",
+                        "blurb": "Bitrate in bits per second",
                         "conditionally-available": false,
                         "construct": false,
                         "construct-only": false,

video/rav1e/src/rav1enc/imp.rs

@@ -278,7 +278,7 @@ impl ObjectImpl for Rav1Enc {
                     .build(),
                 glib::ParamSpecInt::builder("bitrate")
                     .nick("Bitrate")
-                    .blurb("Bitrate")
+                    .blurb("Bitrate in bits per second")
                     .minimum(0)
                     .default_value(DEFAULT_BITRATE)
                     .mutable_ready()