docs: fix typo in captures_read #1807

Job	Run time
msrv	2m 4s
miri-regex-automata	10m 24s
test (stable, ubuntu-latest, stable)	9m 19s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	4m 50s
test (stable-mips, ubuntu-latest, stable, mips64-unknown-linux-gnuabi64)	10m 56s
test (beta, ubuntu-latest, beta)	7m 9s
test (nightly, ubuntu-latest, nightly)	8m 55s
test (macos, macos-latest, stable)	8m 12s
test (win-msvc, windows-latest, stable)	11m 57s
test (win-gnu, windows-latest, stable-x86_64-gnu)	7m 20s
rustfmt	21s
testfull-regex	6m 48s
testfull-regex-automata	14m 15s
testfull-regex-capi	44s
testfull-regex-syntax	3m 38s
	1h 46m 52s

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