Chapter 12.3 Improvement: Text to code inconsistency (repeated use of expect)

[intervall-daniel]

• 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://doc.rust-lang.org/book/ch12-03-improving-error-handling-and-modularity.html

Description of the problem:
In the text it says: "Fourth, we use expect repeatedly to handle different errors, ...", but actually at this point expect is only used once.

Suggested fix:
Drop that partial sentence, like this: "Fourth, if the user runs our program without specifying ...".