Fix instructions for Zig/collatz-conjecture

The task doesn’t mention ComputationError at all, so I (as a new Zig user) was a bit confused about the compiler’s error messages.

Exercism practice exercises are built on a test driven development model. The prose isn’t going to call out everything that is required to solve the exercise. It is expected that you run tests, examine the test output, and update your code accordingly.

… and that you read the tests.

Understood, thanks.

To late :( I thought it was a bug. (But solved it as soon as i saw this thread. )

I did find this a little confusing :person_shrugging: