usage: add a comma between short and long command line options

[Federico Bruni]

While updating the translation, I spot a missing comma between the short
and the long option, in Documentation/usage/external.itely:

@item -nd --no-articulation-directions
do not convert directions (@code{^}, @code{_} or @code{-}) for
articulations, dynamics, etc.

Of course, no need to create an issue for this.
I don't want to edit the english manuals, even for this minor errors. So
please fix it and push it.

Thanks
Federico