update CHANGELOG.md for #3705

Author: bhcleek

CHANGELOG.md

@@ -63,6 +63,9 @@ IMPROVEMENTS:
   [[GH-3697]](https://github.com/fatih/vim-go/pull/3697)
 * Add support for `gopls rename`.
   [[GH-3698]](https://github.com/fatih/vim-go/pull/3698)
+* Use a double border around doc popup.
+  [[GH-3705]](https://github.com/fatih/vim-go/pull/3705)
+
 
 BUG FIXES:
 * Update [impl](https://github.com/josharian/impl) source path after its default branch was changed from master to main.