SSD1680: corrected pen type

libraries/pico_graphics/pico_graphics_pen_2bit.cpp

@@ -4,7 +4,7 @@ namespace pimoroni {
 
     PicoGraphics_Pen2Bit::PicoGraphics_Pen2Bit(uint16_t width, uint16_t height, void *frame_buffer, uint16_t layers)
     : PicoGraphics(width, height, layers, frame_buffer) {
-        this->pen_type = PEN_3BIT;
+        this->pen_type = PEN_2BIT;
         if(this->frame_buffer == nullptr) {
             this->frame_buffer = (void *)(new uint8_t[buffer_size(width, height)]);
         }