rustdoc book doc example error

[csditchfield]

closes #117036

This is the minimal change required to make the second what-to-include.md example valid.
Another more modern solution could be considered:

/// Example
/// ```rust
/// let fortytwo = "42".parse::<u32>()?;
/// println!("{} + 10 = {}", fortytwo, fortytwo+10);
/// #     Ok::<(), <u32 as std::str::FromStr>::Err>(())
/// ```

[rustbot]

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[notriddle]

Thanks a bunch!

[notriddle]

@bors r+ rollup

[bors]

📌 Commit 6596270 has been approved by notriddle

It is now in the queue for this repository.