Wrong spelling of existing

Change "exising" to "existing"

Change-Id: I23b5f3f7bd358b5a5ebe0a4ec712f739549e8771
diff --git a/tempest/api/compute/admin/test_agents.py b/tempest/api/compute/admin/test_agents.py
index d2b3a81..9f7bbae 100644
--- a/tempest/api/compute/admin/test_agents.py
+++ b/tempest/api/compute/admin/test_agents.py
@@ -53,7 +53,7 @@
         if rand_key in kwargs:
             # NOTE: The rand_name is for avoiding agent conflicts.
             # If you try to create an agent with the same hypervisor,
-            # os and architecture as an exising agent, Nova will return
+            # os and architecture as an existing agent, Nova will return
             # an HTTPConflict or HTTPServerError.
             kwargs[rand_key] = data_utils.rand_name(kwargs[rand_key])
         return kwargs