Make centering of STRING_SPLASH work

Marlin/dogm_lcd_implementation.h

@@ -118,7 +118,7 @@ static void lcd_implementation_init()
 
   // Show splashscreen
   int off = (u8g.getWidth() - START_BMPWIDTH) / 2;
-  int txtX = (u8g.getWidth() - sizeof(STRING_SPLASH) - 1) / 2;
+  int txtX = (u8g.getWidth() - (sizeof(STRING_SPLASH) - 1)*5) / 2; // 5 is fontwidth in pixel
   int txtY = u8g.getHeight() - 10;
 	u8g.firstPage();
 	do {