Luhn: Fix instructions

Exercism Building Exercism
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I was looking at Discord help and realized that a recent PR I authored instructs the reader to check if credit card numbers are valid. However, the tests check a variety of numbers, not just credit card numbers. I’m not sure how I missed that, but in any case, this appears to be my fault. Apologies.

Here’s the proposed fix. Change L3 from

Determine whether a credit card number is valid according to the [Luhn formula][luhn].

to

Determine whether a number is valid according to the [Luhn formula][luhn].

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I also took the opportunity to make a minor revision to the instructions for clarity and consistency.

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So the discord help thread about the luhn exercise is here (OP claims that instructions are misleading. Maybe they are not clear enough, but they don’t look misleading to me)

I updated the instructions in order to address that issue too. Let me know what you think.

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I’ve updated the story as well. According to Wikipedia, the luhn formula:

was designed to protect against accidental errors, not malicious attacks

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Question: Since the exercise tests more than just credit card numbers, would it make sense to use a different type of number in the second example — perhaps one with an odd number of digits — to avoid having an example that largely repeats the same procedure as the first?

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I’ve changed the second example to use an invalid Canadian SIN (9 digits)

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