Inverse example from Threshold 2 Channels page needs to be modified to work

[hkmanching]

Describe the bug
Typo for function on inverse example on https://plantcv.readthedocs.io/en/stable/threshold_dual_channels/

To Reproduce
NA

Expected behavior
Should be pcv.threshold.dual_channels, but is written as pcv.threshold.threshold_2_channels

Local environment (please complete the following information):
NA

Additional context
Add any other context about the problem here.

[HaleySchuhl]

@hkmanching thanks for bringing this typo to our attention!