Correct misleading error message in locale mismatch assert.

Don't advise people to use non-existent (or at least not publicly accessible)
wxSetLocale(), they should just create wxLocale objects instead.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73930 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775