STYLE: Replace "the the" with "the" in itkGradientImageFilter.h

Follow-up to pull request #4532
commit b4f0bb8
STYLE: Replace "the the" with "the" in comments

Author: N-Dekker

Modules/Filtering/ImageGradient/include/itkGradientImageFilter.h

@@ -227,7 +227,7 @@ class ITK_TEMPLATE_EXPORT GradientImageFilter : public ImageToImageFilter<TInput
   // when computing the derivatives.
   bool m_UseImageDirection{ true };
 
-  // allow setting the the m_BoundaryCondition
+  // allow setting the m_BoundaryCondition
   std::unique_ptr<ImageBoundaryCondition<TInputImage, TInputImage>> m_BoundaryCondition{
     std::make_unique<ZeroFluxNeumannBoundaryCondition<TInputImage>>()
   };