Update Yacht's config.json to include the second required solution source file

[IndiGan]

The exercise relies on the Yacht.kt file and this one depends on YachtCategory.kt file (which declares the utility enum class).

As of now, only Yacht.kt is included in the solution files array inside config.json.
This causes the two following errors :

• HELP.md is not mentioning the correct command to submit the exercise (beginners using the Exercism CLI may not easily understand why tests are passing locally but not on the server) : exercism submit src/main/kotlin/Yacht.kt (currently) instead of exercism submit src/main/kotlin/Yacht.kt src/main/kotlin/YachtCategory.kt (corrected)
• The shortcut command (exercism submit without any arguments) uses only Yacht.kt instead of passing both files

With the addition of YachtCategory.kt in the solution files, these two issues are fixed.

[github-actions]

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