docs: zh_CN/vm: fix a typo for page reporting ReST file

typo:
	_free_page_reporting.rst -> free_page_reporting.rst

Fixes: 9859b277 ("docs/zh_CN: add vm frontswap translation")
Fixes: adeacecb ("docs/zh_CN: add free_page_reporting translation")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Yanteng Si<siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/c7db8f975f461eaebd7ea7489ff9666d89e121c6.1656234456.git.mchehab@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/vm/free_page_reporting.rst

 .. include:: ../disclaimer-zh_CN.rst
 
-:Original: Documentation/vm/_free_page_reporting.rst
+:Original: Documentation/vm/free_page_reporting.rst
 
 :翻译:
 

Documentation/translations/zh_CN/vm/frontswap.rst

-:Original: Documentation/vm/_free_page_reporting.rst
+:Original: Documentation/vm/free_page_reporting.rst
 
 :翻译: