Join broken line and add colon

As per KOTP's suggestions on track raku :

exercism/raku#669

exercises/perfect-numbers/description.md

@@ -1,11 +1,10 @@
 # Description
 
-Determine if a number is perfect, abundant, or deficient based on
-Nicomachus' (60 - 120 CE) classification scheme for positive integers.
+Determine if a number is perfect, abundant, or deficient based on Nicomachus' (60 - 120 CE) classification scheme for positive integers.
 
 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