update license info of showdown
update license info of showdown

ref: https://github.com/showdownjs/showdown/blob/master/license.txt

dhenson02/cdnjs,viskin/cdnjs,viskin/cdnjs,dhenson02/cdnjs,dhenson02/cdnjs,viskin/cdnjs,dhenson02/cdnjs,viskin/cdnjs,viskin/cdnjs