docs: Update README.ja.md

[eltociear]

Pull request checklist

• The PR has a proper title. Use Semantic Commit Messages. (No more branch-name title please)
• Make sure you are requesting the right branch: dev.
• Make sure this is ready to be merged into the relevant branch. Please don't create a PR and let it hang for a few days.
• Ensure all tests are passing.
• Ensure linting is passing.

PR type

• chore

Description

• fix markdown.

Screenshot

• Please include a screenshot if applicable