add semicolon to Tutorial-Mutations.md

[jayonthegreen]

Missing the missing semicolons. :)

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[wincent]

Thanks @holdonnn. Will get this merged in as soon as the bot confirms that the CLA is signed.

[wincent]

Going to close this one due to inactivity, and go ahead and add the missing semi-colon anyway. Thanks for pointing out the omission, @holdonnn.