schema: adds missing modbus field

./stats/app_layer/error/modbus

etc/schema.json

@@ -3783,6 +3783,9 @@
                                 "krb5_udp": {
                                     "$ref": "#/$defs/stats_applayer_error"
                                 },
+                                "modbus": {
+                                    "$ref": "#/$defs/stats_applayer_error"
+                                },
                                 "mqtt": {
                                     "$ref": "#/$defs/stats_applayer_error"
                                 },