chore: readme examples to es6

[jajugoguma]

Please check if the PR fulfills these requirements

• It's submitted to right branch according to our branching model
• It's right issue type on title
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• It does not introduce a breaking change or has description for the breaking change

Description

• Change examples in README to ES6
• Prettier table of contents in README

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Thank you for your contribution to TOAST UI product. 🎉 😘 ✨

[lja1018]

리뷰완료합니다.

https://nhn.github.io/tui.date-picker/latest/Calendar#event-draw
https://nhn.github.io/tui.date-picker/latest/DatePicker#event-change
https://nhn.github.io/tui.date-picker/latest/DatePicker#event-close
https://nhn.github.io/tui.date-picker/latest/DatePicker#event-draw
https://nhn.github.io/tui.date-picker/latest/DatePicker#event-open
https://nhn.github.io/tui.date-picker/latest/DateRangePicker#event-change:end
https://nhn.github.io/tui.date-picker/latest/DateRangePicker#event-change:start
여기도 화살표함수로 바꾸는거 어떤가요?

[jungeun-cho]

확인했습니다.
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