docs: align subject case rule in readme #3159

[lxow456]

Description

 Update the scope-case rule in docs/reference-rules.md and @commitlint/config-conventional/README.md to be same.

Motivation and Context

 To fix #3159.

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!