Update Japanese assignment.md

[kamegoro]

Description

Assignment.ja.md was not translated into Japanese, so it has been updated.

Fixes # (issue)

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

[kamegoro]

@microsoft-github-policy-service agree

[github-actions[bot]]

This PR has not seen any action for a while! Closing for now, but it can be reopened at a later date.