correct prepare_default_command to _prepare_default_command and remove redundant definition in Subdispatch/DashedStyle

There appear to be typos in `Subdispatch.pm` and `Subdispatch/DashedStyle.pm`, wherein _prepare_default_command is mistakenly called as `prepare_default_command`. 

This results in a runtime failure when using `Subdispatch` (without `DashedStyle`), and invoking a command without a subcommand.  Normally this would result in the default subcommand being run, but instead one gets an error complaining that `prepare_default_command` is not defined.

Additionally, `Subdispatch/DashedStyle` implements a `prepare_default_command` subroutine, which (if my theory about the typo is correct) is redundant with the `_prepare_default_command` that is inherited from `Cmd.pm`.