Clarify in the write-up that disemvowel figures out cases via the number of arugments

[NicMcPhee]

Several people asked about how to tell which of the three cases they're doing for the disemvowel problem. It would be nice to improve the write-up to make it clearer that the script should count how many arguments it's given, and then use that to decide which of the three cases they're handling at this point.