Update analogWrite.adoc

arduino#979 
corecting the analog pin example.

Language/Functions/Analog IO/analogWrite.adoc

@@ -83,7 +83,7 @@ Sets the output to the LED proportional to the value read from the potentiometer
 [source,arduino]
 ----
 int ledPin = 9;      // LED connected to digital pin 9
-int analogPin = 3;   // potentiometer connected to analog pin 3
+int analogPin = A3;  // potentiometer connected to analog pin 3
 int val = 0;         // variable to store the read value
 
 void setup() {