typo fix (#1194)

Author: kyuan-mongodb

source/includes/options-k8s-shared.yaml

@@ -688,7 +688,7 @@ description: |
 
   If you provide more than one value for
   :setting:`spec.security.authentication.modes`, you must also specify a
-  value for :setting:`spec.security.authentication.agent.mode`.
+  value for :setting:`spec.security.authentication.agents.mode`.
 
 ---
 program: _shared