Fix PR links in changelog

CHANGELOG.md

@@ -5,9 +5,12 @@
   * Note that Signal doesn't support real location messages, so they're just
     bridged as links. The link template is configurable.
 * Fixed bridging long text messages from Signal
-  (thanks to [@maltee1] in [#509]).
+  (thanks to [@maltee1] in [#506]).
 * Improved handling of ping timeouts in Signal websocket.
 
+[#504]: https://github.com/mautrix/signal/pull/504
+[#506]: https://github.com/mautrix/signal/pull/506
+
 # v0.6.0 (2024-04-16)
 
 * Updated to libsignal v0.44.0.