chore(deps): update rust crate clap_mangen to v0.2.20 #76

Job	Run time
cargo clippy -- -D warnings (ubuntu-latest)	18s
cargo check (ubuntu-latest)	19s
cargo clippy -- -D warnings (macOS-latest)	20s
cargo fmt --all -- --check	13s
Code Coverage (ubuntu-latest, unix)	2m 0s
cargo check (macOS-latest)	21s
cargo clippy -- -D warnings (windows-latest)	8s
Code Coverage (macos-latest, macos)	1m 42s
Code Coverage (windows-latest, windows)	2m 6s
cargo check (windows-latest)	1m 43s
cargo test (ubuntu-latest)	27s
cargo test (macOS-latest)	27s
cargo test (windows-latest)	1m 47s
	11m 51s

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