Fix broken doc links in the demo app widget gallery (#3441)

* Closes <https://github.com/emilk/egui/issues/3439>

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -109,7 +109,7 @@ impl WidgetGallery {
             date,
         } = self;
 
-        ui.add(doc_link_label("Label", "label,heading"));
+        ui.add(doc_link_label("Label", "label"));
         ui.label("Welcome to the widget gallery!");
         ui.end_row();
 
@@ -121,7 +121,7 @@ impl WidgetGallery {
         );
         ui.end_row();
 
-        ui.add(doc_link_label("TextEdit", "TextEdit,text_edit"));
+        ui.add(doc_link_label("TextEdit", "TextEdit"));
         ui.add(egui::TextEdit::singleline(string).hint_text("Write something here"));
         ui.end_row();
 
@@ -149,10 +149,7 @@ impl WidgetGallery {
         });
         ui.end_row();
 
-        ui.add(doc_link_label(
-            "SelectableLabel",
-            "selectable_value,SelectableLabel",
-        ));
+        ui.add(doc_link_label("SelectableLabel", "SelectableLabel"));
         ui.horizontal(|ui| {
             ui.selectable_value(radio, Enum::First, "First");
             ui.selectable_value(radio, Enum::Second, "Second");
@@ -216,7 +213,11 @@ impl WidgetGallery {
         #[cfg(feature = "chrono")]
         {
             let date = date.get_or_insert_with(|| chrono::offset::Utc::now().date_naive());
-            ui.add(doc_link_label("DatePickerButton", "DatePickerButton"));
+            ui.add(doc_link_label_with_crate(
+                "egui_extras",
+                "DatePickerButton",
+                "DatePickerButton",
+            ));
             ui.add(egui_extras::DatePickerButton::new(date));
             ui.end_row();
         }
@@ -237,7 +238,7 @@ impl WidgetGallery {
         });
         ui.end_row();
 
-        ui.add(doc_link_label("Plot", "plot"));
+        ui.add(doc_link_label_with_crate("egui_plot", "Plot", "plot"));
         example_plot(ui);
         ui.end_row();
 
@@ -273,8 +274,16 @@ fn example_plot(ui: &mut egui::Ui) -> egui::Response {
 }
 
 fn doc_link_label<'a>(title: &'a str, search_term: &'a str) -> impl egui::Widget + 'a {
+    doc_link_label_with_crate("egui", title, search_term)
+}
+
+fn doc_link_label_with_crate<'a>(
+    crate_name: &'a str,
+    title: &'a str,
+    search_term: &'a str,
+) -> impl egui::Widget + 'a {
     let label = format!("{title}:");
-    let url = format!("https://docs.rs/egui?search={search_term}");
+    let url = format!("https://docs.rs/{crate_name}?search={search_term}");
     move |ui: &mut egui::Ui| {
         ui.hyperlink_to(label, url).on_hover_ui(|ui| {
             ui.horizontal_wrapped(|ui| {