Wrong function in code example in section 4.5

[andreqts]

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

URL to the section(s) of the book with this problem: https://rust-book.cs.brown.edu/ch04-05-ownership-recap.html

Description of the problem:

In the 4th code example, the function make_document is called, but it is not defined in the example nor in previous ones.

Suggested fix: Replace with new_document, function that is defined in previous code block, and seem to work fine, and it seems that were the original intention.

[willcrichton]

Thanks, fixed in #83.