Bug 34052: Use root relative URL for subscription-detail.pl

Bug 34052: Use root relative URL for subscription-detail.pl

This patch replaces a same directory relative URL with a root
relative URL, so that the navigation to subscription-detail.pl
works correctly after deleting a serial.

To test:
0. Apply patch
1. Go to http://localhost:8081/cgi-bin/koha/serials/serials-home.pl
2. Click "New subscription"
3. Click "Search for a vendor"
4. Click "OK"
5. Click "Choose"
6. Click "Search for record"
7. Type "e" into "Keyword:" box
8. Click "Search"
9. Click "Choose" on top result
10. Click "Next"
11. Fill in "Serials planning" however you like, and click
"Test prediction pattern"
12. Click "Save subscription"
13. Click "Serial collection" in left nav menu
14. Click "Delete selected issues"
15. Click "Yes, delete"
16. Click the link under the "Subscription num." table column
17. Note that you're taken to the subscription detail page

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>