Argon2: Fix docs about minimum value of `m_cost` (#459) #356

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 37s
build (1.65.0, wasm32-unknown-unknown)	3m 5s
build (stable, thumbv7em-none-eabi)	36s
build (stable, wasm32-unknown-unknown)	32s
test (1.65.0)	3m 1s
test (stable)	38s
minimal-versions / minimal-versions	1m 42s
	12m 11s

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