[docs] Fix the scroll functionality of the mini drawer demo (#13460)
[docs] Fix the scroll functionality of the mini drawer demo (#13460)

* Fix the scroll functionality of the mini drawer demo

In this commit, we are changing the CSS for the mini drawer demo so to 
separate the vertical scroll of the drawer to be independent from the 
window scroll.

Credits for @oliviertassinari that came up with these changes.

Fixes #13412

* remove one classe

callemall/material-ui,Kagami/material-ui,mbrookes/material-ui,Kagami/material-ui,oliviertassinari/material-ui,lgollut/material-ui,Kagami/material-ui,mbrookes/material-ui,rscnt/material-ui,kybarg/material-ui,Kagami/material-ui,mbrookes/material-ui,rscnt/material-ui,callemall/material-ui,kybarg/material-ui,oliviertassinari/material-ui,kybarg/material-ui,kybarg/material-ui,callemall/material-ui,callemall/material-ui,allanalexandre/material-ui,mui-org/material-ui,rscnt/material-ui,oliviertassinari/material-ui,mui-org/material-ui,mui-org/material-ui