'Expect argument for flag' shows Debug instead of Display output

[seanmonstar]

Example:

$ cargo build --features
Expected argument for flag 'Long("features")' but reached end of arguments.

Usage:
    cargo build [options]

I suspect this is a case of using {:?} instead of {} in the error message.

[alexcrichton]

I believe that this will be fixed in the next docopt upgrade which is currently blocked, so I'm going to close this as it should be fixed regardless soon!