Update thread.c

Changed kmain to lk_main

kernel/thread.c

@@ -770,7 +770,7 @@ void thread_sleep(lk_time_t delay) {
 /**
  * @brief  Initialize threading system
  *
- * This function is called once, from kmain()
+ * This function is called once, from lk_main()
  */
 void thread_init_early(void) {
     int i;