Bug 37814: Fix incorrect use of __() in .tt files

Bug 37814: Fix incorrect use of __() in .tt files

This patch corrects instances of the double-underscore function being
used in .tt files where the single-underscore function should
be used instead.

To test, apply the patch and update a translation, e.g. fr-FR:

  > gulp po:update --lang fr-FR

- Open the corresponding .po file for the affected  strings, in this
  case misc/translator/po/fr-FR-staff-prog.po
- Confirm that the strings are now in the .po file for translation. You
  should find these lines:

  - koha-tmpl/intranet-tmpl/prog/en/modules/members/alert-subscriptions.tt:
    "Are you sure you want to unsubscribe %s from email alerts for %s?"
  - koha-tmpl/intranet-tmpl/prog/en/modules/members/memberentrygen.tt
    "Click to expand this section"

- Check fr-FR-opac-bootstrap.po for this line:
  - koha-tmpl/opac-tmpl/bootstrap/en/modules/opac-alert-subscriptions.tt:
    "Are you sure you want to unsubscribe %s from email alerts for %s?"

Sponsored-by: Athens County Public Libraries
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>