docs: translations: zn_CN: irq-affinity.rst: add a missing extension

Without .rst extension, ./scripts/documentation-file-ref-check will produce
a warning, and auto-generating cross-references by automarkup.py may not
work.

Fixes: 8137a49e ("docs/zh_CN: Modify the translator tag and fix the wrong word")
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/22eadbe9fc99fffbd440714f2e4423a5519392a3.1634630486.git.mchehab+huawei@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/core-api/irq/irq-affinity.rst

 .. include:: ../../disclaimer-zh_CN.rst
 
-:Original: Documentation/core-api/irq/irq-affinity
+:Original: Documentation/core-api/irq/irq-affinity.rst
 
 :翻译: