docs/ja_JP/index: update section title in Japanese

Update section title "Japanese Translation" in Japanese.
This change is to keep consistency with other translations.
Signed-off-by: Kosuke Fujimoto <fujimotokosuke0@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Tsugikazu Shibata <shibata@linuxfoundation.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: linux-doc@vger.kernel.org
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220425124853.8347-1-fujimotokosuke0@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/ja_JP/index.rst

@@ -5,7 +5,7 @@
 	\kerneldocCJKon
 	\kerneldocBeginJP{
 
-Japanese translations
+日本語訳
 =====================
 
 .. toctree::