guides: Fix translation of tooltip.

Merged into 5.0.x because this is not a new string, the string is currently
in english in the UI, the string has been translated as some point since
it is in the po files. So it at least give a chance to have this string
translated for 5.0.x.

src/gui/guides.c

@@ -738,7 +738,7 @@ GtkWidget *dt_guides_popover(dt_view_t *self, GtkWidget *button)
 
   GtkWidget *contrast = darktable.view_manager->guides_contrast = dt_bauhaus_slider_new_action(DT_ACTION(self), 0, 1, 0.005, 0.5, 3);
   dt_bauhaus_widget_set_label(contrast, N_("guide lines"), N_("contrast"));
-  gtk_widget_set_tooltip_text(contrast, N_("set the contrast between the lightest and darkest part of the guide overlays"));
+  gtk_widget_set_tooltip_text(contrast, _("set the contrast between the lightest and darkest part of the guide overlays"));
   dt_bauhaus_slider_set(contrast, dt_conf_get_float("darkroom/ui/overlay_contrast"));
   gtk_box_pack_start(GTK_BOX(vbox), contrast, TRUE, TRUE, 0);
   g_signal_connect(G_OBJECT(contrast), "value-changed", G_CALLBACK(_settings_contrast_changed), NULL);
@@ -942,4 +942,3 @@ void dt_guides_update_popover_values()
 // vim: shiftwidth=2 expandtab tabstop=2 cindent
 // kate: tab-indents: off; indent-width 2; replace-tabs on; indent-mode cstyle; remove-trailing-spaces modified;
 // clang-format on
-