Fix shell linter error

Did not correctly account for shell glob matching.

.local/share/qutebrowser/userscripts/pagetopdf.sh

@@ -4,7 +4,7 @@
 # use title of current page / selected text as filename
 filename=${QUTE_SELECTED_TEXT:-$QUTE_TITLE}
 # revert to default name if nothing else is set; remove special chars
-filename=$(echo ${filename:-downloaded.pdf} | sed 's/[<>,/]/_/g')
+filename=$(echo "${filename:-downloaded.pdf}" | sed 's/[<>,/]/_/g')
 
 # print to pdf
 echo "print --pdf '$QUTE_DOWNLOAD_DIR/${1:-$filename}.pdf'" >>"$QUTE_FIFO"