DOC: add server name at rate-limit sessions example

(cherry picked from commit 7282d8eb8b5af097c95ad2e82f861415cf0ed3c9)
(cherry picked from commit 3817f023627473fbdf378d88b04a5557d2442afb)
diff --git a/doc/configuration.txt b/doc/configuration.txt
index a84e951..33917e9 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -5395,7 +5395,7 @@
             mode tcp
             bind :25
             rate-limit sessions 10
-            server 127.0.0.1:1025
+            server smtp1 127.0.0.1:1025
 
   Note : when the maximum rate is reached, the frontend's status is not changed
          but its sockets appear as "WAITING" in the statistics if the