Bug 30688: Error in path to CSS background image

Bug 30688: Error in path to CSS background image

This patch corrects the path of an image specified as the background to
a couple of elements in the OPAC. The incorrect path was causing 404
errors.

To test, apply the patch and rebuild the OPAC CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- If necessary, activate one or more cover image services in
  Administration, e.g. OPACAmazonCoverImages, OPACLocalCoverImages,
  and/or OpenLibraryCovers.
- In the OPAC, locate a bibliographic record which has a cover image
  from one or more of the services you activated.
- On the detail page for that record, the cover should load correctly.
- In the browser console you should see no errors related to a file not
  found: /opac-tmpl/bootstrap/img/spinner-small.gif
- In the browser console, submit this JavaScript snippet:

  $(".cover-image").remove();

  The cover image should disappear, revealing the "spinner" icon behind
  it.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>