perfect-numbers: 'natural number' -> 'positive integer'

There is no convention whether zero is a natural number, so let's avoid
the issue.

For example, see:
- https://mathworld.wolfram.com/NaturalNumber.html
- https://en.wikipedia.org/wiki/Natural_number
- https://math.stackexchange.com/questions/283/is-0-a-natural-number

The previous wording of
```
  "description": "Zero is rejected (not a natural number)",
```
could be especially jarring for some tracks. For example, Nim defines
`Natural` and `Positive` types, and `Natural` includes 0. See:
- https://nim-lang.github.io/Nim/system.html#Natural

Co-authored-by: Sascha Mann <[email protected]>

exercism/problem-specifications#1691

exercises/perfect-numbers/README.md

@@ -1,9 +1,9 @@
 # Perfect Numbers
 
 Determine if a number is perfect, abundant, or deficient based on
-Nicomachus' (60 - 120 CE) classification scheme for natural numbers.
+Nicomachus' (60 - 120 CE) classification scheme for positive integers.
 
-The Greek mathematician [Nicomachus](https://en.wikipedia.org/wiki/Nicomachus) devised a classification scheme for natural numbers, identifying each as belonging uniquely to the categories of **perfect**, **abundant**, or **deficient** based on their [aliquot sum](https://en.wikipedia.org/wiki/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
+The Greek mathematician [Nicomachus](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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
 
 - **Perfect**: aliquot sum = number
   - 6 is a perfect number because (1 + 2 + 3) = 6