Revise user scale validation description

[JayVDZ]

Updated validation statement for user scale in README.

Description

Closes #

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (no functional change)
• Other (describe below)

Testing

• dotnet build JIM.sln succeeds with zero errors
• dotnet test JIM.sln passes
• New functionality has tests (unit, workflow, or integration as appropriate)
• Manually tested in a local development environment

Documentation

• Public documentation updated (docs/); page(s):
• Engineering reference documentation updated (engineering/) where a design/architecture doc would otherwise be stale
• No documentation needed

Screenshots / output (if applicable)

Checklist

• My commit messages are descriptive and reference the relevant Issue (if any)
• My code follows the conventions in docs/DEVELOPER_GUIDE.md
• User-facing text uses British English (en-GB)
• This PR does not include security-sensitive information (real credentials, customer data, internal hostnames)

Additional context