Update ArduinoCellular.cpp

Fixes an error in the sendSMS function that made it impossible to send SMS.

Author: WolfRorDev

Diff for: src/ArduinoCellular.cpp

@@ -127,6 +127,8 @@ Time ArduinoCellular::getCellularTime(){
 
 
 void ArduinoCellular::sendSMS(String number, String message){
+    modem.sendAT("+CMGF=1"); 
+    modem.waitResponse(1000);
     modem.sendAT(GF("+CMGS=\""), number, GF("\""));
     if (modem.waitResponse(GF(">")) != 1) { }
     modem.stream->print(message);  // Actually send the message