Fixed comment mentioned #29

examples/MQ-135-ALL/MQ-135-ALL.ino

@@ -98,10 +98,10 @@ void loop() {
   float Aceton = MQ135.readSensor(); // Sensor will read PPM concentration using the model, a and b values set previously or from the setup
   Serial.print("|   "); Serial.print(CO); 
   Serial.print("   |   "); Serial.print(Alcohol);
-  // Note: 200 Offset for CO2 source: https://github.com/miguel5612/MQSensorsLib/issues/29
+  // Note: 400 Offset for CO2 source: https://github.com/miguel5612/MQSensorsLib/issues/29
   /*
   Motivation:
-  We have added 200 PPM because when the library is calibrated it assumes the current state of the
+  We have added 400 PPM because when the library is calibrated it assumes the current state of the
   air as 0 PPM, and it is considered today that the CO2 present in the atmosphere is around 400 PPM.
   https://www.lavanguardia.com/natural/20190514/462242832581/concentracion-dioxido-cabono-co2-atmosfera-bate-record-historia-humanidad.html
   */