Error message for --gcthreads is inconsistent (typo?) with the --help description

[bvdmitri]

julia --help shows

 --gcthreads=M[,N]         Use M threads for the mark phase of GC and N (0 or 1) threads for the concurrent sweeping phase of GC.
                           M is set to half of the number of compute threads and N is set to 0 if unspecified.

If you accidentally mess up the order of arguments, the error message says the following:

➜  julia git:(master) ./julia --gcthreads 1,4
ERROR: julia: --gcthreads=<n>,<m>; n must be 0 or 1

It is confusing and for me its clearly a typo.
Note that positions of M and N are switched.
The error shows that n must be 0 or 1, while according to the error message itself the <n> is the first one and it is already set to 1. I'll make a small PR for this.