Use backticks on inline math

exercises/perfect-numbers/description.md

@@ -4,7 +4,7 @@ Determine if a number is perfect, abundant, or deficient based on Nicomachus' (6
 
 The Greek mathematician [Nicomachus][nicomachus] devised a classification scheme for positive integers, identifying each as belonging uniquely to the categories of **perfect**, **abundant**, or **deficient** based on their [aliquot sum][aliquot-sum].
 The aliquot sum is defined as the sum of the factors of a number not including the number itself.
-For example, the aliquot sum of 15 is: (1 + 3 + 5) = 9
+For example, the aliquot sum of 15 is `(1 + 3 + 5) = 9`
 
 - **Perfect**: aliquot sum = number
   - 6 is a perfect number because (1 + 2 + 3) = 6