Fix osx path to find the "sml" command.

The defaul root installation directory of the "Standard ML of New Jersey" is
/usr/local/smlnj, the "sml" command is found under the "bin" directory wich is
not automatically found by SublimeText.

Ref: http://www.smlnj.org/dist/working/110.75/NOTES/MACOSXINSTALL