docs(display): use lv_display_delete_refr_timer to delete display timer

[jakergrossman]

While not a valid LVGL program, the following snippet illustrates the problem with the current
example:

// main.c
#include <lvgl.h>

int main(void) {
	lv_display_t* disp = (void*)0xFEEDBEEF;

	lv_timer_delete(disp->refr_timer);
	disp->refr_timer = NULL;
}

Because lv_display_t is an opaque typedef:

../main.c: In function ‘main’:
../main.c:6:29: error: invalid use of incomplete typedef ‘lv_display_t’ {aka ‘struct _lv_display_t’}
    6 |         lv_timer_delete(disp->refr_timer);
      |                             ^~
../main.c:7:13: error: invalid use of incomplete typedef ‘lv_display_t’ {aka ‘struct _lv_display_t’}
    7 |         disp->refr_timer = NULL;
      |             ^~