ch13-03: Use correct variable name

[brunojppb]

Hey Rust folks 👋 ,

First of, thanks for this amazing book! Now onto this tiny fix.
The code where this paragraph references to uses file_path instead of filename as the variable name:

book/listings/ch13-functional-features/listing-13-20/src/lib.rs

Lines 23 to 25 in a60f431

	let file_path = match args.next() {
	Some(arg) => arg,
	None => return Err("Didn't get a file path"),

This fix should use the matching variable name according to the Rust code.

[carols10cents]

Thank you, closing as a duplicate of #3457 though. Please check existing PRs and issues before opening new ones in the future. Thanks!

[brunojppb]

Thank you, closing as a duplicate of #3457 though. Please check existing PRs and issues before opening new ones in the future. Thanks!

Thank you @carols10cents 🙌
I actually checked the opened PRs and didn't find one addressing this issue already. (the one you referenced was created on a later date)
But is good to keep that in mind.

[carols10cents]

Whoops, sorry about that, I thought I had the list sorted and picked the oldest.