Update test_tutorials.py docstring to match NOTEBOOK_PATH's default value (#825)

* fix default NOTEBOOK_PATH value to satisfy the documentation

* Revert "fix default NOTEBOOK_PATH value to satisfy the documentation"

This reverts commit c857ac0.

* update test_tutorials.py docstring

Author: pedrorrivero

utils/test/test_tutorials.py

@@ -20,8 +20,8 @@
 """Helper for running the notebooks as unit tests.
 
 Convenience script for running the notebooks as individual `unittest` tests
-using the standard Python facilites. By default, only the notebooks under
-`reference/` are automatically discovered (can be modified via the
+using the standard Python facilites. By default, all notebooks under
+`qiskit/` are automatically discovered (can be modified via the
 `NOTEBOOK_PATH` variable).
 
 The test can be run by using the regular unittest facilities from the root