Missing link in Filtering.tex

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.