Bug 14202: Unexpected parameter for window.open in marc21_linking_section.pl

Bug 14202: Unexpected parameter for window.open in marc21_linking_section.pl

The builder of this plugin includes a window.open call with the name
parameter: \"marc21_field_7\"+ event.data.id +\"\"
Note that this is not the name of the new window.
This trivial patch adjusts this unusual name by tag_editor like other
plugins.

Test plan:
Attach marc21_linking_section to 773$9.
Click on the Tag editor button.
If it opens, it still works :)
Note that the name of the popup is still:
    Koha > Cataloguing > plugin for links

Signed-off-by: Fredreic Demians <f.demians@tamil.fr>
  Plugin still working, with a valid pop-pup window id.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>