Change simple code in book

[ccqpein]

In Get started with Druid of book, the simple code:

fn main() -> Result<(), PlatformError> {
    AppLauncher::with_window(WindowDesc::new(build_ui)).launch(())?;
    Ok(())
}

doesn't pass the compiler. Change it to:

fn main() -> Result<(), PlatformError> {
    AppLauncher::with_window(WindowDesc::new(build_ui())).launch(())?;
    Ok(())
}

will work

[SecondFlight]

You have to add to the list of links at the bottom of the file for the [#1698] and [@ccqpein] shortcuts to work

[ccqpein]

Thanks for notifying me, let me change it now.

[ccqpein]

Just done. Please check it out.

[SecondFlight]

Thanks! I'll merge this tomorrow morning if there are no other comments.

[memoryruins]

This change causes the example code to no longer compile with 0.7, although the book instructs the user to use 0.7 right above the example. New users have ran into this on discord.

I realize users of master branch can run into this without this change #1637, but the book doesn't currently specify using a git version. Should this be reverted?

[SecondFlight]

Based on this Zulip conversation, I've opened a PR (#1732) to update the book to use the latest git version. This was an oversight when I reviewed this PR. Sorry for the confusion it caused!