Fix issue #548: git checkout command copy the file for you

User runs that command, and git does the copy for user.

Author: YueLinHo

book/02-git-basics/sections/undoing.asc

@@ -122,7 +122,7 @@ You can see that the changes have been reverted.
 [IMPORTANT]
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 It's important to understand that `git checkout -- <file>` is a dangerous command.
-Any changes you made to that file are gone – you just copied another file over it.
+Any changes you made to that file are gone – Git just copied another file over it.
 Don't ever use this command unless you absolutely know that you don't want the file.
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