Update Pythagorean triplet README.md with missing constraint

[TheBestJohn]

Added missing constraint to the Pythagorean triplet problem that is outlined in the linked Project Euler problem.

[coriolinus]

Hi @TheBestJohn, and thank you for contributing this! That does look like a good and useful constraint to add.

Rust gets its problem specifications from the problem-specifications repo, and one of our CI checks ensures that the READMEs for each exercise here correspond with the README there. We therefore can't successfully merge this PR until it is merged upstream, in that repo.

Are you willing to open another PR, making the same change in that repository?

[TheBestJohn]

Sure thing. Thanks for the info.

[coriolinus]

With exercism/problem-specifications#1365 merged, I'm restarting Travis.