secrecy: fix `Secret<T>` reference in README.md (#1249) #810

Job	Run time
build (1.60.0, armv7a-none-eabi)	1m 58s
test (ubuntu-latest, 1.60.0)	1m 46s
build (1.60.0, thumbv7em-none-eabi)	2m 22s
build (1.60.0, wasm32-unknown-unknown)	1m 51s
test (ubuntu-latest, stable)	18s
build (stable, armv7a-none-eabi)	10s
test (macos-latest, 1.60.0)	2m 7s
build (stable, thumbv7em-none-eabi)	10s
test (macos-latest, stable)	12s
build (stable, wasm32-unknown-unknown)	10s
test (windows-latest, 1.60.0)	3m 19s
test (windows-latest, stable)	1m 33s
	15m 56s

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