Fixed minor formatting issue in introduction.md

[ninadkhire]

While using Exercism for practice found that the ```swift ... ``` is visible in the introduction guide, which I thought is due to a formatting issue caused due to the surrounding tag ~~~~exercism/warrning ~~~~. Hence removed the tag to format it similarly as other code snippets.

Hope it will be helpful. Thanks.

[github-actions]

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[IsaacG]

Avoid nesting blocks.
+cc @exercism/cross-track-maintainers for review