Fix property case at contents.css

[tlewin]

Fix the following warning:

▲ [WARNING] "overflow-X" is not a known CSS property [unsupported-css-property]

    node_modules/@toast-ui/editor/dist/toastui-editor.css:962:2:
      962 │   overflow-X: hidden;
          │   ~~~~~~~~~~
          ╵   overflow-x

  Did you mean "overflow-x" instead?

Please check if the PR fulfills these requirements

• It's the right issue type on the 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 a description of the breaking change

Description

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

[boazblake]

please merge this to master