kbuild: edit explanation of clean-files variable

Commit 97659181 (kbuild Update documentation of clean-files and clean-dirs)
changed the example for the clean-files variable, but left the explanation
unchanged. Edit the explanation to match the example.

Signed-off-by: Jeffrey Rogers <jeffreydavidrogers@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>