docs/ja_JP/index: update section title in Japanese

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.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>