fix: correct class name for text field start/end styles

[chrisdholt]

Description

This PR fixes an unusual regression which I thought was already fixed. Fixing it again.

Motivation & context

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[EisenbergEffect]

I remember reviewing a PR like this too...maybe we fixed it in one design system but not the other?

[chrisdholt]

I remember reviewing a PR like this too...maybe we fixed it in one design system but not the other?

Super odd - It's broken in both...I'm just not pushing a fix for MSFT because that package is about to drop in a subsequent PR :)