Fixed minor typos in comments of task 6.09

ch_6/task_09/main.c

@@ -51,7 +51,7 @@ void stack_print (cmd_t *head);
  *                                                                            *
  *      @param head             Pointer to pointer to the first stack element *
  *      @param command          Pointer to the desired command                *
- *      @retval OK              Pushing successfull                           *
+ *      @retval OK              Pushing successful                            *
  *      @retval FULL            Pushing failed -- stack is full               *
  ******************************************************************************/
 retval_t stack_push (cmd_t **head, char *command) {
@@ -94,8 +94,8 @@ retval_t stack_push (cmd_t **head, char *command) {
  *      the next command from the stack.                                      *
  *                                                                            *
  *      @param head             Pointer to pointer to the first stack element *
- *      @param command          Pointer to the readback command               *
- *      @retval OK              Popping successfull                           *
+ *      @param command          Pointer to the read back command              *
+ *      @retval OK              Popping successful                            *
  *      @retval FULL            Popping failed -- stack is empty              *
  ******************************************************************************/
 retval_t stack_pop (cmd_t **head, char *command) {