docs: remove scope-case rule #3157

[lxow456]

Description

 Removed scope-case rule from @commitlint/config-conventional/README.md to update the documentation.

Motivation and Context

 I referenced to the #828 . The pr modified @commitlint/config-conventional/index.js. Verification of scope-case has been removed. But the documentation was not updated together.
 And to fix #3157 .

Usage examples

 Removed the following contents from the documentation.

scope-case

• condition: scope is in case value
• rule: always
• level: error

'lowerCase'

echo "fix(SCOPE): some message" # fails
echo "fix(scope): some message" # passes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[escapedcat]

Thanks!