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 :)

[lordofhyphens]

Easy PR to https://github.com/slic3r/Slic3r/blob/master/xs/src/libslic3r/PrintConfig.cpp#L1727 :)

[VuWhite]

PR: #5189