Update introduction.md Interest is Interesting exercise

Programming C#
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Added a ; to the do loop as it is needed to compile. Might be confusing for beginners when they replicate the example code and it doesn’t work because of a missing ;.
Pull request can be found here: Update introduction.md by joMarsch · Pull Request #2203 · exercism/csharp · GitHub

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+cc @ErikSchierboom

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Thanks! Merged.