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

[ee7]

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

Note, the other exercises that mention "natural" are:

difference-of-squares/description.md
1:Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.
3:The square of the sum of the first ten natural numbers is
6:The sum of the squares of the first ten natural numbers is
10:ten natural numbers and the sum of the squares of the first ten
11:natural numbers is 3025 - 385 = 2640.

difference-of-squares/metadata.yml
2:blurb: "Find the difference between the square of the sum and the sum of the squares of the first N natural numbers."

prime-factors/description.md
1:Compute the prime factors of a given natural number.

prime-factors/metadata.yml
2:blurb: "Compute the prime factors of a given natural number."

pythagorean-triplet/description.md
1:A Pythagorean triplet is a set of three natural numbers, {a, b, c}, for

spiral-matrix/description.md
3:The matrix should be filled with natural numbers, starting from 1

sum-of-multiples/description.md
4:If we list all the natural numbers below 20 that are multiples of 3 or 5,