docs: Fix typo in `?articulation_points` #2644

Job	Run time
Build source R package	5m 12s
Sanitizer	24m 33s
Coverage	5m 5s
Check windows-latest (release)	10m 51s
Check macOS-latest (release)	9m 28s
Check ubuntu-20.04 (release)	6m 35s
Check ubuntu-20.04 (devel)	5m 45s
Check ubuntu-20.04 (oldrel-1)	5m 55s
Check ubuntu-20.04 (oldrel-2)	6m 24s
Check ubuntu-20.04 (oldrel-3)	5m 47s
Check ubuntu-20.04 (oldrel-4)	6m 7s
	1h 31m 42s

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