Fix a typo

Author: topecongiro

src/bin/rustfmt.rs

@@ -68,7 +68,7 @@ impl CliOptions {
         let mut options = CliOptions::default();
         options.skip_children = matches.opt_present("skip-children");
         options.verbose = matches.opt_present("verbose");
-        let unstable_features = matches.opt_present("unstable_features");
+        let unstable_features = matches.opt_present("unstable-features");
         let rust_nightly = option_env!("CFG_RELEASE_CHANNEL")
             .map(|c| c == "nightly")
             .unwrap_or(false);