Add missing type annotation for docs #97

Job	Run time
Check (stable, ubuntu-latest)	18s
Check (stable, macOS-latest)	26s
Check (stable, windows-latest)	44s
Check (beta, ubuntu-latest)	16s
Check (beta, macOS-latest)	19s
Check (beta, windows-latest)	42s
Check (nightly, ubuntu-latest)	15s
Check (nightly, macOS-latest)	17s
Check (nightly, windows-latest)	40s
Clippy (stable)	15s
Clippy (beta)	15s
Clippy (nightly)	15s
Test Suite (stable, ubuntu-latest)	23s
Test Suite (stable, macOS-latest)	23s
Test Suite (stable, windows-latest)	49s
Test Suite (beta, ubuntu-latest)	21s
Test Suite (beta, macOS-latest)	23s
Test Suite (beta, windows-latest)	1m 0s
Test Suite (nightly, ubuntu-latest)	18s
Test Suite (nightly, macOS-latest)	23s
Test Suite (nightly, windows-latest)	1m 4s
	9m 46s

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