Bug 29593: Fix wrong tag in GET /public/libraries spec

Bug 29593: Fix wrong tag in GET /public/libraries spec

This makes the documentation look untidy.

To test:
1. Open the api/v1/swagger/paths/libraries.json file with your favourite
   editor.
=> FAIL: All routes, but this one, have tags: ['library']
2. Apply this patch
=> SUCCESS: All routes have tags: ['libraries']
3. swagger.yaml too!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>