Fix a document: eframe::start_web (#3026)

In `0.22.0`,  `eframe::start_web` has been replaced with `eframe::WebRunner`, which also installs a nice panic hook (no need for `console_error_panic_hook`).

crates/eframe/src/lib.rs

@@ -7,7 +7,7 @@
 //! To learn how to set up `eframe` for web and native, go to <https://github.com/emilk/eframe_template/> and follow the instructions there!
 //!
 //! In short, you implement [`App`] (especially [`App::update`]) and then
-//! call [`crate::run_native`] from your `main.rs`, and/or call `eframe::start_web` from your `lib.rs`.
+//! call [`crate::run_native`] from your `main.rs`, and/or use `eframe::WebRunner` from your `lib.rs`.
 //!
 //! ## Usage, native:
 //! ``` no_run