docs: Migration fixed typo (fix: react-bootstrap#3966) (react-bootstrap#3972)

Author: mxschmitt

www/src/pages/migrating.mdx

@@ -74,7 +74,7 @@ via context to allow intermediate nesting of child components
 
 ### InputGroup
 
-- removed InputGroupButton, and InputGroupAddon
+- removed InputGroup.Button, and InputGroup.Addon
 - added InputGroup.Prepend, InputGroup.Append, InputGroup.Text, InputGroup.Checkbox, InputGroup.Radio
 
 ### Badge & Label