Bug 38469: Replace single quotes with double quotes to prevent XSS

Bug 38469: Replace single quotes with double quotes to prevent XSS

This change replaces single quotes with double quotes to prevent XSS
for particular operations on the circ returns page.

Test plan:
0. Apply the patch
1. Go to http://localhost:8081/cgi-bin/koha/circ/returns.pl?print_slip=1&reserve_id=1
2. Note that a pring slip is generated
(you may need to allow popups)

3. To test the XSS is patched, try the proof-of-concept from the
bug report

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Baptiste Wojtkowski <baptiste.wojtkowski@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: wainuiwitikapark <wainuiwitikapark@catalyst.net.nz>