update CHANGELOG.md for #3633

Author: bhcleek

CHANGELOG.md

@@ -48,6 +48,8 @@ IMPROVEMENTS:
   [[GH-3567]](https://github.com/fatih/vim-go/pull/3567)
 * Add support for the Go 1.21's new builtin functions.
   [[GH-3578]](https://github.com/fatih/vim-go/pull/3578)
+* Add syntax support for the toolchain directive in go.mod.
+  [[GH-3633]](https://github.com/fatih/vim-go/pull/3633)
 
 BUG FIXES:
 * Update [impl](https://github.com/josharian/impl) source path after its default branch was changed from master to main.