LED error sequence was changed, lights-of-dead gif should be updated

**Description**

The documentation page for the [LED error sequence](https://github.com/ARMmbed/Debugging-docs/blob/master/Docs/Debugging/lights_of_dead.md) contains a gif showing the LED error sequence.

The sequence was recently changed to be 4 fast pulses followed by 4 slow pulses in [mbed-os 5.6](https://github.com/ARMmbed/mbed-os/pull/4920).

打开 Docs/Debugging/lights_of_dead.md 并检查其中嵌入的 LED 错误序列 GIF，然后查看链接的 mbed-os 5.6 更改以了解新的时序。更新 GIF，使其显示 4 个快速脉冲，随后显示 4 个慢速脉冲，并验证文档页面能正确显示它。

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