doc: fix typo in module-level doc #1830

Job	Run time
testfull-regex-automata	17m 53s
msrv	2m 42s
miri-regex-automata	24m 33s
rustfmt	7s
testfull-regex	8m 17s
test (stable, ubuntu-latest, stable)	6m 52s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	5m 57s
test (stable-mips, ubuntu-latest, stable, mips64-unknown-linux-gnuabi64)	11m 45s
test (beta, ubuntu-latest, beta)	9m 29s
test (nightly, ubuntu-latest, nightly)	8m 36s
test (macos, macos-latest, stable)	13m 59s
test (win-msvc, windows-latest, stable)	10m 53s
test (win-gnu, windows-latest, stable-x86_64-gnu)	8m 19s
testfull-regex-capi	46s
testfull-regex-syntax	4m 6s
	2h 14m 14s

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