fix: change default send_queue_limit to 500 (XRPLF#4867)

Clients subscribed to `transactions` over WebSocket are being
disconnected because the traffic exceeds the default `send_queue_limit`
of 100.

This commit changes the default configuration, not the default in code.

Fix XRPLF#4866

cfg/rippled-example.cfg

@@ -1641,6 +1641,7 @@ port = 6006
 ip = 127.0.0.1
 admin = 127.0.0.1
 protocol = ws
+send_queue_limit = 500
 
 [port_grpc]
 port = 50051
@@ -1651,6 +1652,7 @@ secure_gateway = 127.0.0.1
 #port = 6005
 #ip = 127.0.0.1
 #protocol = wss
+#send_queue_limit = 500
 
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