Input: fix bug in example code

The input example driver uses BTN_0 in the later stages of the
example, so this changes the interrupt routine to match.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Documentation/input/input-programming.txt

@@ -22,7 +22,7 @@ static struct input_dev *button_dev;
 
 static void button_interrupt(int irq, void *dummy, struct pt_regs *fp)
 {
-	input_report_key(button_dev, BTN_1, inb(BUTTON_PORT) & 1);
+	input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1);
 	input_sync(button_dev);
 }