[docs] Update transform function in the sx prop sizing docs

[ZeeshanTamboli]

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

Previews:

• https://deploy-preview-33850--material-ui.netlify.app/system/the-sx-prop/#sizing
• https://deploy-preview-33850--material-ui.netlify.app/system/sizing/#supported-values

[mui-bot]

No bundle size changes

Generated by :no_entry_sign: dangerJS against 8c8fd5702fcc5eab3efb2115dc3b0422ca65361e

[ZeeshanTamboli]

Can you share an issue about what is the problem and why the change was made? 0% or 0px doesn't seem that important in my opinion. Also, we would need to add tests if this actually creates a problem.

There is no issue. Just saw that the transform function in the implementation and the docs is different. Check for 0 was added in #28219. Tests were also covered there.

It's just a minor thing really. Basically, docs matching the code.