Corrected the return type of the digitalPotWrite in the SPI/DigitalPotControl example

Fixes arduino#1063

Author: Fede85

libraries/SPI/examples/DigitalPotControl/DigitalPotControl.ino

@@ -60,12 +60,12 @@ void loop() {
 
 }
 
-int digitalPotWrite(int address, int value) {
+void digitalPotWrite(int address, int value) {
   // take the SS pin low to select the chip:
   digitalWrite(slaveSelectPin,LOW);
   //  send in the address and value via SPI:
   SPI.transfer(address);
   SPI.transfer(value);
   // take the SS pin high to de-select the chip:
   digitalWrite(slaveSelectPin,HIGH); 
-}
+}