Removed one more utils reference in the README and fixed directory name

Signed-off-by: Carlo Costino <carlo.costino@gsa.gov>

tests/notifications_utils/clients/test_redis.py

@@ -0,0 +1,7 @@
+from notifications_utils.clients.redis import rate_limit_cache_key
+
+
+def test_rate_limit_cache_key(sample_service):
+    assert rate_limit_cache_key(sample_service.id, "TEST") == "{}-TEST".format(
+        sample_service.id
+    )