Use `Frame::NONE` in docstring (#7396)

- Replaced deprecated `egui::Frame::none()` with `egui::Frame::NONE`

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* Closes <https://github.com/emilk/egui/issues/7391>
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---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/containers/frame.rs

@@ -46,7 +46,7 @@ use epaint::{Color32, CornerRadius, Margin, MarginF32, Rect, Shadow, Shape, Stro
 ///
 /// ```
 /// # egui::__run_test_ui(|ui| {
-/// egui::Frame::none()
+/// egui::Frame::NONE
 ///     .fill(egui::Color32::RED)
 ///     .show(ui, |ui| {
 ///         ui.label("Label with red background");