Bug 25696: Correct "Test prediction pattern" HTML button

Bug 25696: Correct "Test prediction pattern" HTML button

This patch corrects the closing tag for the "Test prediction pattern"
button from "</a>" to "</button>"

Test plan:
1. Apply the patch
2. Create a serial subscription from a new or existing bib record
3. Choose any frequency, numbering pattern, and subscription start
date you choose (ie fill out all mandatory subscription fields)
4. Click "Test prediction pattern"
5. Note that the prediction pattern appears on the right of the screen

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>