Update 99bottles.rb

We set a variable ("start") to 99, the song begins with 99 bottles of beer.

We then set a variable to "start"

We don't want an infinite loop, we just want the program to loop until we are out of bottles of beer.

As long as there are greater than or equal to 1 bottle of beer on the wall we want the loop to continue, it should end after we take the last bottle of beer.

Every time we take a bottle of beer away, the next time the program runs it should adjust to what we have left..  num = num - 1