docs: zh_CN/devicetree: fix typos

docs: zh_CN/devicetree: fix typos

The devicetree directory was using wrong case:

	Devicetree/ -> devicetree/

Fixes: a17b0169f29b ("docs/zh_CN: add devicetree index translation")
Fixes: c56481299df3 ("docs/zh_CN: add devicetree usage-model translation")
Fixes: 09d4466d3f3c ("docs/zh_CN: add devicetree of_unittest translation")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Yanteng Si<siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/b4ca010ef223ab9cb76e8caa0a3cde734e0b6b2f.1656234456.git.mchehab@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>