docs: Improve description of destination_dir and keep_files

#324 (comment)

Author: peaceiris

README.md

@@ -333,7 +333,7 @@ For example:
 
 ### ⭐️ Keeping existing files `keep_files`
 
-By default, existing files in the publish branch are removed before adding the ones from publish dir. If you want the action to add new files but leave existing ones untouched, set the optional parameter `keep_files` to `true`.
+By default, existing files in the publishing directory (or `destination_dir` if you use) will be removed. If you want the action to add new files but leave existing ones untouched, set the optional parameter `keep_files` to `true`.
 
 Note that users who are using a Static Site Generator do not need this option in most cases. Please reconsider your project structure and building scripts, or use a built-in feature of a Static Site Generator before you enable this flag.