secrecy: fix `Secret<T>` reference in README.md (#1249) #853

Job	Run time
build (1.81.0, armv7a-none-eabi)	26s
build (1.81.0, thumbv7em-none-eabi)	26s
build (1.81.0, wasm32-unknown-unknown)	29s
build (stable, armv7a-none-eabi)	20s
build (stable, thumbv7em-none-eabi)	39s
build (stable, wasm32-unknown-unknown)	21s
test (ubuntu-latest, 1.81.0)	1m 21s
test (ubuntu-latest, stable)	26s
test (macos-latest, 1.81.0)	40s
test (macos-latest, stable)	42s
test (windows-latest, 1.81.0)	1m 26s
test (windows-latest, stable)	1m 57s
	9m 13s

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