Update Adafruit_PCD8544.cpp

Formula was simply wrong (IMHO), I was wrote the following function in Adafruit_GFX.cpp (had to add the ```virtual uint8_t getPixel(int8_t x, int8_t y);``` in Adafruit_GFX.h):

```c
void Adafruit_GFX::shiftScreen() {
   /* Shift screen 6 lines up */
	cursor_y -= 6;
	for (int8_t ally = 0; ally<_height-6; ally++)
	{
		for (int8_t allx = 0; allx<_width; allx++)
		{
			drawPixel(allx, ally, getPixel(allx, ally+6));
		}
	}
	fillRect(0, _height-6, _width, 6, 0);
}
```

The result was garbage. After my patch, everything works as expected.