Fix broken link

Closes https://github.com/emilk/egui/issues/1643

egui_demo_lib/src/easy_mark/easy_mark_editor.rs

@@ -194,7 +194,7 @@ Goals:
 2. easy to learn
 3. similar to markdown
 
-[The reference parser](https://github.com/emilk/egui/blob/master/egui/src/experimental/easy_mark_parser.rs) is \~250 lines of code, using only the Rust standard library. The parser uses no look-ahead or recursion.
+[The reference parser](https://github.com/emilk/egui/blob/master/egui_demo_lib/src/easy_mark/easy_mark_parser.rs) is \~250 lines of code, using only the Rust standard library. The parser uses no look-ahead or recursion.
 
 There is never more than one way to accomplish the same thing, and each special character is only used for one thing. For instance `*` is used for *strong* and `-` is used for bullet lists. There is no alternative way to specify the *strong* style or getting a bullet list.