Merge pull request #600 from thorbenk/config_help_cmd

fix documentation of config help

Author: marcusklaas

README.md

@@ -55,7 +55,7 @@ screen, for example.
 
 Rustfmt is designed to be very configurable. You can create a TOML file called
 rustfmt.toml, place it in the project directory and it will apply the options
-in that file. See `cargo run --help-config` for the options which are available,
+in that file. See `cargo run -- --config-help` for the options which are available,
 or if you prefer to see source code, [src/config.rs].
 
 By default, Rustfmt uses a style which (mostly) confirms to the