[V4] Fixes #17953: Update migration docs of .btn-default replacement

[V4 Docs] Update migration docs to point out the .btn-default replacement.

docs/migration.md

@@ -29,7 +29,6 @@ Here are the big ticket items you'll want to be aware of when moving from v3 to
 
 ### Components
 
-- Dropped button `.btn-default` class.
 - Dropped panels, thumbnails, and wells for a new all-encompassing component, cards.
 - Dropped the Glyphicons icon font. If you need icons, some options are:
   - the upstream version of [Glyphicons](http://glyphicons.com/)
@@ -83,7 +82,7 @@ New to Bootstrap 4 is the Reboot, a new stylesheet that builds on Normalize with
 
 ### Buttons
 
-- Dropped the `.btn-default` class and additionaly add a new button class named `.btn-secondary`.
+- Renamed `.btn-default` to `.btn-secondary`.
 
 ### Grid system