Ch 3-05 Calling out loop label format

[MasterCash]

• 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:

• https://github.com/rust-lang/book/blob/main/src/ch03-05-control-flow.md#repeating-code-with-loop

Description of the problem:

• Currently it doesn't describe that the ' is required for loop labels (e.g. 'counting_up). It would be nice if this was called out explicitly as a requirement for the name of a loop label.

Suggested fix:

• It would be nice to have a line or two describing that the ' is required for the naming of a loop label