Volumetric checkbox incorrect information, easy fix

[arthurwolf]

Describe the bug

When hovering over the "volumetric extrusion" option/checkbox, it states that volumetric is an experiemental Marlin feature.

Smoothie has has volumetric like, forever, and it's not experimental at all. So that would probably be a good thing to fix for users using Slic3r and Smoothie together.

Easy fix :)