Update PubSubClient.h

[anushavenkatesh1]

Increased the buffer size of MQTT_MAX_PACKET_SIZE from 256 to 500

[hmueller01]

Why should we increase the default packet size? If you need a bigger buffer use a compiler define -D MQTT_MAX_PACKET_SIZE 500 or use PubSubClient::setBufferSize(500) within your code.