Fixed typo in AdaptiveThreshold

[rold2007]

Prerequisites

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Description

I saw a clear typo in the code. So I added a new test which would validate the change. All the images used in the tests were previously grayscale so that the typo would go unnoticed. That being said, using adaptive thresholding on a color image is not a usual use case.

[codecov]

Codecov Report

Merging #1362 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1362   +/-   ##
=======================================
  Coverage   82.77%   82.77%           
=======================================
  Files         690      690           
  Lines       30975    30975           
  Branches     3511     3511           
=======================================
  Hits        25641    25641           
  Misses       4613     4613           
  Partials      721      721           

Flag	Coverage Δ
#unittests	82.77% <100.00%> (ø)

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Impacted Files	Coverage Δ
...Binarization/AdaptiveThresholdProcessor{TPixel}.cs	100.00% <100.00%> (ø)

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[brianpopow]

@rold2007 thank you for the fix!