readme: fix ascon-hash256 link #50

Job	Run time
set-msrv / set-msrv	6s
minimal-versions / minimal-versions	36s
test (1.85.0)	32s
build (stable, thumbv7em-none-eabi)	15s
build (1.85.0, thumbv7em-none-eabi)	23s
build (1.85.0, wasm32-unknown-unknown)	23s
build (stable, wasm32-unknown-unknown)	22s
test (stable)	37s
	3m 14s

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