Passing the path parameter by reference

(Couldn't find a better way to ask a question, sorry!)

At some point in your article [the `file_path` parameter](https://github.com/BurntSushi/rust-error-handling-case-study/blob/master/src/main.rs#L97) becomes `&Option<..>` instead of simply `Option<..>`. Why?

Start at src/main.rs#L97 and read the linked article section that discusses the file_path parameter. Compare the surrounding Rust code with the explanation to determine why the parameter is passed as a reference to an Option. Done means documenting that explanation clearly in the relevant article or project documentation.

Written by the indexing model from the issue text.