k256: fix comment (#929) #1950

Job	Run time
benches (1.65.0)	6m 26s
benches (stable)	2m 16s
build (1.65.0, thumbv7em-none-eabi)	4m 23s
build (1.65.0, wasm32-unknown-unknown)	5m 44s
build (stable, thumbv7em-none-eabi)	1m 8s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	6m 49s
doc	38s
build (stable, wasm32-unknown-unknown)	1m 21s
cross (armv7-unknown-linux-gnueabihf, stable)	2m 8s
cross (aarch64-unknown-linux-gnu, 1.65.0)	7m 12s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	6m 21s
cross (aarch64-unknown-linux-gnu, stable)	2m 6s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	4m 26s
cross (powerpc-unknown-linux-gnu, 1.65.0)	5m 49s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 0s
cross (powerpc-unknown-linux-gnu, stable)	2m 5s
test (x86_64-unknown-linux-gnu, stable)	2m 17s
	1h 7m 9s

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