Fix link to eframe (#130)

The name of the primary branch in the upstream Repo is master not main.
This PR fixes the link. Alternatively the name of the primary branch
could just be changed to main as well.

src/app.rs

@@ -102,7 +102,7 @@ fn powered_by_egui_and_eframe(ui: &mut egui::Ui) {
         ui.label(" and ");
         ui.hyperlink_to(
             "eframe",
-            "https://github.com/emilk/egui/tree/main/crates/eframe",
+            "https://github.com/emilk/egui/tree/master/crates/eframe",
         );
         ui.label(".");
     });