book: fix typo #2041

Job	Run time
minimal	27s
check-cfg	23s
msrv	2m 0s
docs	23s
quickchecking	18s
rustfmt-clippy	32s
test-book	9s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 11s
test-expectations (ubuntu-latest)	1m 0s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 59s
test-expectations (macos-latest)	2m 37s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	2m 8s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 55s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 53s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 41s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 14s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 24s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	2m 41s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	2m 55s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 35s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	2m 51s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 22s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	4m 0s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 17s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 35s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 12s
test (macos-latest, 9.0, 0, 0, 0)	2m 23s
success	1s
	56m 6s

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