fix(tests): fix docstring for expired KES test

Part of the docstring was accidentally stripped in one of the previous changes.

Author: mkoura

cardano_node_tests/tests/test_kes.py

@@ -144,6 +144,7 @@ def test_expired_kes(
     ):
         """Test expired KES.
 
+        * start local cluster instance configured with short KES period and low number of key
           evolutions, so KES expires soon on all pools
         * refresh opcert on 2 of the 3 pools, so KES doesn't expire on those 2 pools and
           the pools keep minting blocks