Update feature_unicode.md UNICODE_MODE_LINUX -> UC_LNX

[TryAndErr0r]

the defines in the documenation do not exist and throw errors on compilation.

this file https://github.com/qmk/qmk_firmware/blob/b835171008eaeaa992a1b8e390af8bce6f5f0b8f/docs/ja/feature_unicode.md

somewhat documens the change in alias but google shows this file when searched so...

Description

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• [ x] Documentation

Issues Fixed or Closed by This PR

• Fixing names of defines used for configurating unicode input type

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• [ x] My change requires a change to the documentation.
• [ x] I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[fauxpark]

You need to update your repo. See #18898