This PR solves the deprecation warning found here:
https://build.testeng.edx.org/job/edx-platform-python-pipeline-master/warning_5freport_5fall_2ehtml/

Change:
(LibraryLocator(org='org', course='course'), False)
To:
(LibraryLocator(org='org', library='course'), False)
In the file:
openedx/core/djangoapps/content/block_structure/tests/test_signals.py

* LibraryLocator is from the opaque_keys.edx.locator module

See documentation:
https://pythonhosted.org/edx-opaque-keys/opaque_keys.edx.html#opaque_keys.edx.locator.LibraryLocator