fix typo README.ko-kr

[Jeongsuu]

I fixed the typo in Korean translation version README.md

As-Is: "스택바", it means stackbar To-Be: "스낵바", it means snackbar

Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.

Every PR must update the corresponding documentation in the code, and also the readme in english with the following changes.

Pre-launch Checklist

• [x] I updated/added relevant documentation (doc comments with ///).
• [] I added new tests to check the change I am making or feature I am adding, or @jonataslaw said the PR is test-exempt.
• [x] All existing and new tests are passing.