Update instructions to match tests and function signature.

[ColinPitrat]

Tests are verifying that the function works with i64::MAX and u64::MAX so it needs to go beyond 999_999_999_999.

The function signature takes a u64 so saying to test with -1 doesn't make sense.

https://forum.exercism.org/t/say-instructions-are-inconsistent-with-test/18695

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[SleeplessByte]

As mentioned on the forum, the instructions for this particular exercise should not be added or changes directly. @senekor has given the approval for this PR to exist to be considered if it instead changes (adds) instructions.append.md (docs).

Thank you for working on this!

[ColinPitrat]

Ah, there's already an instructions.append.md and indeed, it does display at the end of the instructions:

Extension
Add capability of converting up to the max value for u64: 18_446_744_073_709_551_615.
For hints at the output this should have, look at the last test case.

Somehow I missed it. So no further change is needed I guess.