Fix typo, track function name should be 'track' instead of 'effect'

[tianzhou]

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[skirtles-code]

Thanks for the fix.

This is related to #524. The reason I hadn't made the change proposed here is because there's some subtlety to this and it's arguable as to whether track or effect is really what's intended. However, given it's taking me some time to fix this properly I think it is better for it to say track for now.