Update wkhtmltopdf used to Build PDF Guide

[OpenXTalkPaul]

While re-setting up my laptop to build, now under 64bit ONLY (macOS 10.5.x+, I'm on BigSur 11.6.x now), I had a try at building the Docs "PDF Guide" which gets converted to HTML from a bunch of .md markdown files, and then the HTML is converted to a PDF with "wkhtmltopdf". It failed with the version that was in the Repo which was apparently 32bit (failed for wrong architecture), so I downloaded the latest from: https://wkhtmltopdf.org/downloads.html
The Guide builder is still failing for something else that's missing (looking into it) but wkhtmltopdf is definitely working now.