MINOR: counters: fix a typo in comment

Wrong copy/paste comment, replace listeners/frontends by
servers/backends

This may be backported up to 1.7.
diff --git a/include/haproxy/counters-t.h b/include/haproxy/counters-t.h
index dbd6072..b896c4d 100644
--- a/include/haproxy/counters-t.h
+++ b/include/haproxy/counters-t.h
@@ -63,7 +63,7 @@
 	} p;                                    /* protocol-specific stats */
 };
 
-/* counters used by listeners and frontends */
+/* counters used by servers and backends */
 struct be_counters {
 	unsigned int conn_max;                  /* max # of active sessions */
 	long long    cum_conn;                  /* cumulated number of received connections */