Suggestion: simplify 14.3 Cargo Workspaces member crate initialization

[SichangHe]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • 14.3 workspace
  • ch14-03
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • https://github.com/rust-lang/book/blob/main/src/ch14-03-cargo-workspaces.md

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html

Description of the problem: As per rust-lang/cargo#6378, new crates initialized in workspaces are automatically added into the workspaces' Cargo.toml members list. However, current instructions contain redundant actions to manually add the member crate names into the members list.

Suggested fix: Remove the redundant manual Cargo.toml editing, and instead show and explain how Cargo does that automatically for the user.