Fix layout-padding css

Adds a `,` which was missing in 5caa22b2c444485b3d340cda662d8808b1fc381d when `layout-padding` was readded. Also removes a comment saying `layout-padding` is depreciated since it has been readded.
It appears that [historically](https://github.com/angular/material/blob/32ba2b072036dc63e2c41354aa7f6a9e33852b92/src/core/style/layout.scss#L30-L33) `layout-padding` did have `margin-top` set. I think it'd make sense to either keep it or switch it to `padding-top` for `layout-padding` instead.