der: update link to core ArrayVec RFC #3508

Job	Run time
build (stable, thumbv7em-none-eabi)	15s
build (stable, wasm32-unknown-unknown)	15s
test (stable)	43s
build (1.85.0, wasm32-unknown-unknown)	20s
minimal-versions / minimal-versions	29s
test (1.85.0)	48s
build (1.85.0, thumbv7em-none-eabi)	19s
	3m 9s

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