Sum-of-multiples: Add information to the description.

* Add the word unique to the description.
* Go back to using "below" rather than "up to but not including.
* Remove and between numbers.

exercism/problem-specifications#961
Closes #360

Author: hekrause

exercises/sum-of-multiples/README.md

@@ -1,10 +1,10 @@
 # Sum Of Multiples
 
-Given a number, find the sum of all the multiples of particular numbers up to
+Given a number, find the sum of all the unique multiples of particular numbers up to
 but not including that number.
 
-If we list all the natural numbers up to but not including 20 that are
-multiples of either 3 or 5, we get 3, 5, 6 and 9, 10, 12, 15, and 18.
+If we list all the natural numbers below 20 that are multiples of 3 or 5,
+we get 3, 5, 6, 9, 10, 12, 15, and 18.
 
 The sum of these multiples is 78.