Update CHANGELOG.md

Author: felix920506

CHANGELOG.md

@@ -27,6 +27,8 @@ These changes are available on the `master` branch, but have not yet been releas
 
 - Fix `Enum` options not setting the correct type when only one choice is available.
   ([#2577](https://github.com/Pycord-Development/pycord/pull/2577))
+- Fix `codec` option for `FFmpegOpusAudio` class to make it in line with documentation.
+  ([#2581](https://github.com/Pycord-Development/pycord/pull/2581))
 
 ### Changed