k256: update `Signature::normalize_s` usages #11

Job	Run time
doc	1m 56s
test (1.73.0)	13s
build (1.73.0, thumbv7em-none-eabi)	13s
test (stable)	11s
build (1.73.0, wasm32-unknown-unknown)	15s
build (stable, thumbv7em-none-eabi)	6s
build (stable, wasm32-unknown-unknown)	7s
	3m 1s

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