fix compile warning in demo

fix compile warning in demo

```
In file included from demo/keyboard.c:5:
demo/keyboard.c: In function ‘draw_keyboard’:
demo/../termbox2.h:252:33: warning: unsigned conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} changes value from ‘16777222’ to ‘6’ [-Woverflow]
  252 | #define TB_MAGENTA              0x0006
      |                                 ^
demo/keyboard.c:512:22: note: in expansion of macro ‘TB_MAGENTA’
  512 |     printf_tb(33, 1, TB_MAGENTA | TB_BOLD, TB_DEFAULT, "Keyboard demo!");
```