OAEP example now uses `sha2` re-export #589

Job	Run time
minimal-versions	40s
doc	25s
test (1.65.0)	22m 25s
build (1.65.0, thumbv7em-none-eabi)	7m 13s
build (1.65.0, wasm32-unknown-unknown)	3m 42s
nightly	1m 8s
build (stable, thumbv7em-none-eabi)	18s
test (stable)	7m 7s
build (stable, wasm32-unknown-unknown)	31s
	43m 29s

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