doc: remove repetitive words #2196

Job	Run time
rustfmt	21s
miri-regex-automata	10m 43s
msrv	1m 19s
test (stable, ubuntu-latest, stable)	3m 47s
testfull-regex	3m 50s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 51s
testfull-regex-automata	8m 34s
testfull-regex-capi	32s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 34s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 19s
testfull-regex-syntax	2m 0s
test (beta, ubuntu-latest, beta)	3m 55s
test (nightly, ubuntu-latest, nightly)	3m 55s
test (macos, macos-latest, stable)	4m 3s
test (win-msvc, windows-latest, stable)	6m 2s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 19s
	1h 12m 4s

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