Termination stabilized in 1.61

[ehuss]

• I have checked the latest main branch to see if this has already been fixed
• I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem:

book/src/ch09-02-recoverable-errors-with-result.md

Lines 516 to 520 in ea90bba

	The `main` function may return any types that implement [the
	`std::process::Termination` trait][termination]<!-- ignore -->. As of this
	writing, the `Termination` trait is an unstable feature only available in
	Nightly Rust, so you can’t yet implement it for your own types in Stable Rust,
	but you might be able to someday!

Description of the problem: The Termination trait will be stable in 1.61. The paragraph linked above will likely need to be updated when the book is updated for 1.61.

Suggested fix: I would maybe just delete the last sentence, though I'm not sure if you'd like to say more about it (like an example).

(BTW, let me know if you would prefer to not get these "heads up" issues for future changes. I realize you probably go through the release notes to figure out what to change, so I'm not sure if it is helpful at all.)

[carols10cents]

Thanks! Issues like these are definitely useful, especially with pointers to the exact part of the book that will need to change :)

I am currently working on an updated print version, and I'm trying to decide what version to use for that. If I go with 1.61+, I'll be fixing this soon, if I go with 1.60 or earlier, it'll be a while before I address this.