Fix the separator color in the disabled button with icon example.

[jimmynotjim]

Introduced a bug with incorrect markup. This fixes it.

Changes

• Updated the classname to fix the markup pattern.

Testing

• Checkout the buttons page.

Review

• @Scotchester

Preview this PR without the whitespace changes

Screenshots

Checklist

• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code can be automatically merged (no conflicts)
• [ ] Code follows the standards laid out in the front end playbook
• [ ] Passes all existing automated tests
• [ ] New functions include new tests
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged
• [ ] Visually tested in supported browsers and devices
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)