readme update

README.md

@@ -52,7 +52,7 @@ After power-up, the bootloader starts. All three LEDs are flashed for a second,
     8. Enables write protection of application space if this feature is enabled in the configuration.
     9. After successful in-application-programming, the bootloader launches the application.
 
-- If the button is pressed for more than 4 seconds: the bootloader launches ST's built-in bootloader located in the internal boot ROM (system memory) of the chip. For more information, please refer to [[4]](#references). With this method, the bootloader can be updated or even a full chip re-programming can be performed easily, for instance by connecting the hardware to the computer via USB and using DFU mode [[5, 6]](#references).
+- If the button is pressed for more than 4 seconds: the bootloader launches ST's built-in bootloader located in the internal boot ROM (system memory) of the chip. For more information, please refer to [[5]](#references). With this method, the bootloader can be updated or even a full chip re-programming can be performed easily, for instance by connecting the hardware to the computer via USB and using DFU mode [[6, 7]](#references).
 
 - If the button is kept pressed for more than 9 seconds: the bootloader tries to launch the application located in the flash. This scenario is fully equivalent to the case when the user does not press the button after power-up (see above).