Bump defmt from 0.3.6 to 0.3.8 in /examples/stm32f4-event-printer #85

Job	Run time
stm32f4-event-printer	1m 59s
lib (1.62.0, use_alloc)	2m 53s
lib (1.62.0, use_alloc,defmt)	2m 53s
lib (1.62.0, use_heapless)	2m 45s
lib (1.62.0, use_heapless,defmt)	2m 57s
lib (stable, use_alloc)	1m 43s
lib (stable, use_alloc,defmt)	1m 40s
lib (stable, use_heapless)	1m 29s
lib (stable, use_heapless,defmt)	1m 49s
	20m 8s

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