Book: Fix outdated substring output

[Dracape]

In the NuShell book/Nu Fundamentals/Working with strings/The str command/Substrings (book/working_with_strings.md), the command 'Hello World!' | str substring 4..8''s output is given as o Wo—which would be correct if 8 wasn't included and the output would be of 4 characters, as given in the book.

Running the command internally has shown that 8—in fact inculded in the substring so the correct output shown in the comment should be o Wor (of 5 characters) as it is in the cheat sheet and not o Wo