Update CHANGELOG.md

Author: gordonshieh

CHANGELOG.md

@@ -63,6 +63,7 @@ You can find its changes [documented below](#070---2021-01-01).
 - Build on OpenBSD ([#1993] by [@klemensn])
 - Scope: expose scoped state using state() and state_mut() ([#2082] by [@rjwittams]
 - Tabs: allow getting and setting the tab index of a Tabs widget ([#2082] by [@rjwittams]
+- `Slider` widget now scrollable by mouse wheel ([#2104] by [@gordonshieh])
 
 ### Changed