REGTESTS: server: fix cli_add_server due to previous trace update

Error output for dynamic server creation if invalid lb algo has changed
since previous commit :
MINOR: server: fix doc/trace on lb algo for dynamic server creation

The vtest regex should have been updated has well to match it.
diff --git a/reg-tests/server/cli_add_server.vtc b/reg-tests/server/cli_add_server.vtc
index 8663faf..8bb4cd2 100644
--- a/reg-tests/server/cli_add_server.vtc
+++ b/reg-tests/server/cli_add_server.vtc
@@ -53,7 +53,7 @@
 
 	# invalid load-balancing algo
 	send "experimental-mode on; add server other/s1 ${s1_addr}:${s1_port}"
-	expect ~ "Backend must use a consistent hashing method for load balancing to support dynamic servers."
+	expect ~ "Backend must use a dynamic load balancing to support dynamic servers."
 
 	# invalid mux proto
 	send "experimental-mode on; add server other2/s1 ${s1_addr}:${s1_port} proto h2"