fix Magpi-special editions

[joergi]

somehow the download of special editions are not working
if you go the url https://magpi.raspberrypi.org/books/essentials-bash-vol1/pdf the website wants you first so spend some money, or you can click to direct download.

somehow the code of the downloader needs to be changed to accept these links too.

 printf -v page_url $downloadUrl "$i"
		pdf_url=`curl -sf $page_url | grep "c-link\"" | sed 's/^.*href=\"//' | sed 's/\(\?.*\)\?">.*$//' | sed "s#^\(/.*\)#$siteUrl\1#"`
		wget -N $pdf_url -P $outputDir
		i=$(( i+1 ))