Clarify that the root is a natural number in square-root documentation

[ksnortum]

The instructions in exercises/practice/square-root/.docs/instructions.append.md says that they are "only testing natural numbers," but more clearly, the solutions are only natural numbers. Two is a natural number and its root is real (and irrational).

[github-actions]

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Links to docs:

Pull Request Guide
Markdown Guide
Practice Exercise Spec