escape backslashes in fit function equation

src/gui_qt/fit/function_creation_dialog.py

@@ -472,7 +472,7 @@ class DescWidget(QtWidgets.QWidget):
         stringi = f'class {self.klass_lineedit.text()}:\n' \
                   f'    name = {self.name_lineedit.text()!r}\n' \
                   f'    type = {self.group_lineedit.text()!r}\n' \
-                  f'    equation = {self.eq_lineedit.text()!r}\n'
+                  f"    equation = r'{self.eq_lineedit.text()}'\n"
 
         return stringi