RGBPixel doesn't include subtraction or addition operators

In Book 1, Chapter 4, it says the following ([here](https://itk.org/ITKSoftwareGuide/html/Book1/ITKSoftwareGuide-Book1ch4.html#x38-470004))
> ITK introduces the itk::RGBPixel type as a support for representing the values of an RGB color space. As such, the RGBPixel class embodies a different concept from the one of an itk::Vector in space. For this reason, the RGBPixel lacks many of the operators that may be naively expected from it. In particular, there are no defined operations for subtraction or addition.

According to the Doxygen for [RGBPixel](https://itk.org/Doxygen/html/classitk_1_1RGBPixel.html), there are addition and subtraction operators for the RGBPixel class. I'm not sure the information is correct. Maybe in the past this was not the case however.

Here is what to edit: https://github.com/InsightSoftwareConsortium/ITKSoftwareGuide/blob/master/SoftwareGuide/Latex/Architecture/DataRepresentation.tex#L101