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).

Open Docs/Debugging/lights_of_dead.md and inspect the embedded LED error-sequence GIF, then review the linked mbed-os 5.6 change for the new timing. Update the GIF to show 4 fast pulses followed by 4 slow pulses and verify that the documentation page displays it correctly.

Written by the indexing model from the issue text.