Scrollable: Fix story by declaring field as readonly

[tyxla]

What?

Fixes a warning in the Scrollable story.

Why?

Just fixing a warning in the story.

Found while working on #67574.

How?

We're using the field as a read-only field, so React will complain if there's no onChange or readOnly provided to it.

Testing Instructions

• Open the Scrollable story: http://localhost:50240/?path=/docs/components-experimental-scrollable--docs
• Verify that there is no read-only warning anymore

Testing Instructions for Keyboard

Same

Screenshots or screencast

Error before this PR:

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Co-authored-by: tyxla <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

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[Mamaduka]

Makes sense ✅