[docs] Fix wrong code snippet for overriding styles in theme with a callback value

[ZeeshanTamboli]

• I have followed (at least) the PR section of the contributing guide.

Fixes #32760

Callback as a value is in the CSS class properties not in the styleOverrides key.

In any case, I think, this particular code snippet with the following description doesn't match:

Each component is composed of several different parts.
These parts correspond to classes that are available to the component—see the **CSS** section of the component's API page for a detailed list.
You can use these classes inside the `styleOverrides` key to modify the corresponding parts of the component.

Also this sub-section seems redundant with the section below: Overrides based on props.

I would propose to remove this sub-section which was added in #31997.

Preview: https://deploy-preview-32781--material-ui.netlify.app/material-ui/customization/theme-components/#global-style-overrides

[mui-bot]

No bundle size changes

Generated by 🚫 dangerJS against cd4da69

[danilo-leal]

Nice! I'll leave that up to @siriwatknp as he's definitely more of an expert on this than I am. Thanks for stepping up!

[siriwatknp]

👍 Nice catch! please see my comment before merging.