docs: zh_CN: page_frags.rst: fix a broken reference

typo:
	page_frag.rst -> page_frags.rst

Fixes: f51debc2 ("docs/zh_CN: add vm page_frags translation")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Yanteng Si<siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/a12e18044ddb2ca16a1fed10823d59558d6de405.1656234456.git.mchehab@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/vm/page_frags.rst

-:Original: Documentation/vm/page_frag.rst
+:Original: Documentation/vm/page_frags.rst
 
 :翻译: