Missing link in Filtering.tex

[jhlegarreta]

The lines https://github.com/InsightSoftwareConsortium/ITKSoftwareGuide/blob/6cae7a99926b15d631c47ae211b433230f297a12/SoftwareGuide/Latex/DesignAndFunctionality/Filtering.tex#L118-L119

refer to a section in the SW Guide without using the \ref command.

I'm wondering whether it was not used because linking to the first book from the second book would not work, or else, it was just a simple oversight.

In the Coding Style chapter (which is part of Book 1), I've used links to sections in only Book 1, so I ignore whether links across books would work well.