The previous text suggested to change the scheme in the preferences,
however, the preferences only take effect for new documents. Now, that
we have the "ProjectUnitSystem" in place, the setting for the current
document needs to be changed.
The text is restructured accodring to the UX pattern for warnings:
- What is the problem
- How can I fix it
- What happens if I don't
and includes a link to the wiki for further details.
In addition, the 'US Customary' was removed from the proposed schemes as
suggested during review.
Finally, the button text was decapitalized and aligned with the
style used in other parts of FreeCAD, e.g. the Start page.
