Address issue for deprecated syntax

Change from :FOR to FOR/END to address a new WARNING in
the new robot version.

Sample warning message as mention below.

FOR loop starting on line 26: For loop header
':FOR' is deprecated. Use 'FOR' instead.

Change-Id: I3860386614d8418f10291977654976929f534b69
Signed-off-by: Sushil Singh <susilsi7@in.ibm.com>
diff --git a/redfish/service_root/test_service_root_security.robot b/redfish/service_root/test_service_root_security.robot
index 311a368..59adfa3 100644
--- a/redfish/service_root/test_service_root_security.robot
+++ b/redfish/service_root/test_service_root_security.robot
@@ -188,5 +188,6 @@
 
     FFDC On Test Case Fail
 
-    :FOR  ${item}  IN  @{session_list}
-    \  Redfish.Delete  ${item}
+    FOR  ${item}  IN  @{session_list}
+      Redfish.Delete  ${item}
+    END