demos: Use `demo-root` class in PHP demos as well
demos: Use `demo-root` class in PHP demos as well

Fixing regression introduced in Iaa030055d6da1f17.

Change-Id: Ifa390eb1e0b673ac1d97f0dea3919476e0ae4429

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui