Fix the mlcfg script

[arnaudgolfouse]

The mlcfg script at the root of the repo was failing with the error

error: the following required arguments were not provided:
  --why3-config-file <WHY3_CONFIG_FILE>

Usage: creusot-rustc --why3-config-file <WHY3_CONFIG_FILE> --output-file <OUTPUT_FILE> --span-mode <SPAN_MODE> -- <RUST_FLAGS>...

For more information, try '--help'.

@Armael is that the correct fix ?

[Armael]

Perfect, thanks a lot for the fix!