doc: fix CheckHrpstring::new reference #571

Job	Run time
Prepare	3s
Test - stable toolchain	3m 1s
Docs - stable toolchain (recent)	33s
Check WASM	31s
Test - MSRV	9m 40s
Lint - nightly toolchain (recent)	50s
Docs - nightly toolchain (recent)	35s
Test - nightly toolchain	3m 19s
	18m 32s

Provide feedback