Clarify the sum of multiples problem

It was unclear based on the description that a number should only be counted
once, even if it was the multiple of more than one of the target factors.

Author: kytrinyx

sum-of-multiples.md

@@ -1,5 +1,7 @@
-If we list all the natural numbers below 10 that are multiples of 3 or
-5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
+If we list all the natural numbers below 15 that are multiples of either
+3 or 5, we get 3, 5, 6 and 9, 10, 12, and 15.
 
-Allow the program to be configured to find the sum of multiples of
-numbers other than 3 and 5.
+The sum of these multiples is 60.
+
+Write a program that can find the sum of the multiples of a given set of
+numbers.

sum-of-multiples.yml

@@ -1,4 +1,4 @@
 ---
-blurb: "Write a program that, given a number, can find the sum of all the multiples of 3 or 5 up to but not including that number."
+blurb: "Write a program that, given a number, can find the sum of all the multiples of particular numbers up to but not including that number."
 source: "A variation on Problem 1 at Project Euler"
 source_url: "http://projecteuler.net/problem=1"