Adjust install of ashpd to avoid tokio

Since version 0.10 `ashpd` has shipped with the `tokio` feature enabled by default. This slightly undermines this section of the book where the author is discussing non-tokio ways to run async code.

This commit adjusts the command for adding `ashpd` to exclude `tokio` and use the previously expected `async-std` feature instead.

Without this change, the example code compiles but panics with a `there is no reactor running` message. With this change, the example code runs successfully.

Author: Jwashton

book/src/main_event_loop.md

@@ -213,11 +213,11 @@ Filename: <a class=file-link href="https://github.com/gtk-rs/gtk4-rs/blob/main/b
 Asynchronous functions from the `glib` ecosystem can always be spawned on the `glib` main loop.
 Typically, crates depending on `async-std` or `smol` work as well.
 Let us take `ashpd` for example which allows sandboxed applications to interact with the desktop.
-Per default it depends on `async-std`.
+It can be configured to depend on `async-std`.
 We can add it to our dependencies by running the following command.
 
 ```
-cargo add ashpd --features gtk4
+cargo add ashpd --no-default-features --features "gtk4 async-std"
 ```
 
 You need to use a Linux desktop environment in order to run the following example locally.