Add span element to sample code

[ryan-villena]

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An exercise has the student creating a rule which would affect one out of two <span> elements in the sample code. If the student is working locally having copied the sample code from the top of the page (instead of using the embedded live example), they are missing one of the <span> elements involved in the exercise. This fixes that issue.

No Issue to link to that I could find, possible I could have missed it though. Please let me know if I should have written one; I didn't see that it was required in the contribution guide, and this seemed simple enough a fix.

[rachelandrew]

thanks!