Sync sum-of-multiples docs with problem-specifications (#303)

The sum-of-multiples exercise has been improved upstream in the
problem-specifications repository.

For more context, please see the pull request that updated the exercise:

- exercism/problem-specifications#2231

exercises/practice/sum-of-multiples/.docs/instructions.md

@@ -1,7 +1,18 @@
 # Instructions
 
-Given a number, find the sum of all the unique multiples of particular numbers up to but not including that number.
+Given a list of factors and a limit, add up all the unique multiples of the factors that are less than the limit.
+All inputs will be greater than or equal to zero.
 
-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.
+## Example
 
-The sum of these multiples is 78.
+Suppose the limit is 20 and the list of factors is [3, 5].
+We need to find the sum of all unique multiples of 3 and 5 that are less than 20.
+
+Multiples of 3 less than 20: 3, 6, 9, 12, 15, 18
+Multiples of 5 less than 20: 5, 10, 15
+
+The unique multiples are: 3, 5, 6, 9, 10, 12, 15, 18
+
+The sum of the unique multiples is: 3 + 5 + 6 + 9 + 10 + 12 + 15 + 18 = 78
+
+So, the answer is 78.