Fix warnings in `cargo doc` from invalid link #100

Job	Run time	Billable time
doc	32s	0s
format	13s	0s
check-minimal	24s	0s
check	28s	0s
test (stable, --no-default-features)	33s	0s
test (stable, --no-default-features --features "test")	18s	0s
test (stable, --no-default-features --features "termcolor")	30s	0s
test (stable)	32s	0s
test (stable, --all-features)	29s	0s
test (beta, --no-default-features)	30s	0s
test (beta, --no-default-features --features "test")	35s	0s
test (beta, --no-default-features --features "termcolor")	27s	0s
test (beta)	31s	0s
test (beta, --all-features)	26s	0s
test (nightly, --no-default-features)	23s	0s
test (nightly, --no-default-features --features "test")	26s	0s
test (nightly, --no-default-features --features "termcolor")	24s	0s
test (nightly)	35s	0s
test (nightly, --all-features)	31s	0s
tarpaulin	7s	0s
publish	0s	0s
	8m 54s	0s

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