docs: correct grammar of readme.md

[x] 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

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

nhn / tui.date-picker