update CHANGELOG.md for #2640

Author: bhcleek

CHANGELOG.md

@@ -62,6 +62,7 @@ IMPROVEMENTS:
   [[GH-2605]](https://github.com/fatih/vim-go/pull/2605)
   [[GH-2609]](https://github.com/fatih/vim-go/pull/2609)
   [[GH-2638]](https://github.com/fatih/vim-go/pull/2638)
+  [[GH-2640]](https://github.com/fatih/vim-go/pull/2640)
 * Add a buffer level option, `b:go_fmt_options`, to control formatting options
   per buffer.
   [[GH-2613]](https://github.com/fatih/vim-go/pull/2613)