Fix the example

[mzaffalon]

• bugfix: the method WiFiServer::write was removed in arduino/ArduinoCore-mbed@18b696b

• modify the example keep the connection open until the client quit

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[mzaffalon]

I am new to Arduino and I didn't follow the development. It appears that there are two WiFi libraries, the other one at https://github.com/arduino-libraries/WiFi which is retired as the note at the beginning of the page says, but this is still referred to in the WiFi library reference.
So is it a matter of updating the documentation?