Rename master branch to main (#7034)

For consistency with other repositories, i.e. so I can write `git
checkout main` without worrying which repo I'm browsing.

crates/egui_demo_lib/src/demo/toggle_switch.rs

@@ -121,5 +121,5 @@ pub fn toggle(on: &mut bool) -> impl egui::Widget + '_ {
 }
 
 pub fn url_to_file_source_code() -> String {
-    format!("https://github.com/emilk/egui/blob/master/{}", file!())
+    format!("https://github.com/emilk/egui/blob/main/{}", file!())
 }