chore(librarian): fix typo in state.yaml (#14627)

This PR fixes a typo in `state.yaml` which caused
`tests/unit/test_helpers.py` not to be preserved in
`google-cloud-vision`:
https://github.com/googleapis/google-cloud-python/blob/main/packages/google-cloud-vision/tests/unit/test_helpers.py

Author: parthea

.librarian/state.yaml

@@ -1128,7 +1128,7 @@ libraries:
       - samples/snippets/README.rst
       - tests/system
       - tests/unit/test_decorators.py
-      - tests/unit/tests_helpers.py
+      - tests/unit/test_helpers.py
     remove_regex:
       - packages/google-cloud-vision
     tag_format: '{id}-v{version}'