Update ModbusTCP.h

[hacesoft]

when using NodeMCU 1.0 board. - ESP8266 and latest version of WiFiManager library: 2.0.17 the message is signaled:

modbus-esp8266\src/ModbusTCP.h:21:26: warning: 'WiFiClient WiFiServer::available(uint8_t*)' is deprecated: Renamed to accept(). [-Wdeprecated-declarations]

21 | return available()

after replacing with new method: return WiFiServer::accept(); there are no error messages and communication between NodeMCU and Node-Red works, I personally tried it.