fix typo in README.ja.md

[lv7777]

What did you implement:

fix typo

How did you implement it:

README.ja.md

How can we verify it:

please see README.ja.md

Todos:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO
Is it a breaking change?: NO

[kotakanbe]

Many Thanks :)