Fix sub_registry_with_prefix RustDoc #991

Job	Run time
Test Suite	1m 5s
Clippy	1m 15s
Cross compile (armv7-unknown-linux-gnueabihf)	20s
Cross compile (powerpc-unknown-linux-gnu)	21s
Rustfmt	25s
Check rustdoc intra-doc links	31s
Check	1m 17s
Cross compile (powerpc64-unknown-linux-gnu)	23s
Cross compile (wasm32-unknown-unknown)	23s
	6m 0s

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