Fix missing quotation mark in example

[darthwalsh]

I'm unsure if it's the only issue causing #2780 but the syntax highlighting makes it obviously wrong.

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[PRMerger9]

@darthwalsh : Thanks for your contribution! The author, @Mikejo5000, has been notified to review your proposed change.

[Mikejo5000]

@darthwalsh Thanks for the fix!

[Mikejo5000]

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