fix typo, fix answer, add missing furigana

[ingemarberg]

This pull request contains some more minor fixes as well as furigana for lesson-7/workbook-5 (3rd ed.).

fix typo, fix answer, add missing furigana

• fixed typo in lesson-7/workbook-6 (3rd ed.)
• fixed suggested kana-only answer in lesson-4/workbook-2 (2nd ed.)
• fixed suggested kana-only answer in lesson-4/workbook-2 (3rd ed.)
• fixed suggested kana-only answer in lesson-5/workbook-7 (3rd ed.)
• fixed width of answer field for kana-only answer in lesson-5/grammar-3 (3rd ed.)
• added missing furigana in lesson-7/workbook-5 (3rd ed.)

[ingemarberg]

I forgot to check against the exercises in 2nd Edition when I made the pull request. I have added one more commit to update lesson-4/workbook-2 (2nd ed.) as well.