Fix source string typos mentioned on Crowdin (#15261)

src/Mod/Fem/Gui/TaskFemConstraintDisplacement.cpp

@@ -361,7 +361,7 @@ void TaskFemConstraintDisplacement::addToSelection()
             }
             for (const auto& SubElement : SubElements) {
                 if (SubElement.find(searchStr) == std::string::npos) {
-                    QString msg = tr("Only one type of selection (vertex,face or edge) per "
+                    QString msg = tr("Only one type of selection (vertex, face or edge) per "
                                      "analysis feature allowed!");
                     QMessageBox::warning(this, tr("Selection error"), msg);
                     addMe = false;