Allow --edition 2021 to be passed to rustfmt

This was added to Configurations.md in #4618, but the option wasn't
actually made available. This should let people who are using Rust 2021
on nightly rustc run `cargo fmt` again.

src/bin/main.rs

@@ -689,6 +689,7 @@ fn edition_from_edition_str(edition_str: &str) -> Result<Edition> {
     match edition_str {
         "2015" => Ok(Edition::Edition2015),
         "2018" => Ok(Edition::Edition2018),
+        "2021" => Ok(Edition::Edition2021),
         _ => Err(format_err!("Invalid value for `--edition`")),
     }
 }