Merge rust-bitcoin/rust-secp256k1#757: chore: fix doc typos for keys #1503

Job	Run time
Test 32-bit version	36s
Format - nightly toolchain	11s
Docs - stable toolchain (recent)	18s
Test - 1.63.0 toolchain (minimal)	5m 33s
Prepare	10s
Test - 1.63.0 toolchain (recent)	5m 33s
Test - stable toolchain (minimal)	3m 27s
Test - stable toolchain (recent)	3m 20s
WASM - stable toolchain	1m 46s
Docs - nightly toolchain (recent)	19s
ASAN - nightly toolchain (recent)	1m 30s
Bench - nightly toolchain (recent)	19s
Lint - nightly toolchain (recent)	26s
Test - nightly toolchain (minimal)	2m 34s
no-std - nightly toolchain (recent)	51s
Test - nightly toolchain (recent)	2m 32s
	29m 25s

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