docs: Fix typo in `?articulation_points` #2641

Job	Run time
Build source R package	5m 23s
Sanitizer	17m 19s
Check windows-latest (release)	16m 37s
Coverage	5m 8s
Check macOS-latest (release)	8m 39s
Check ubuntu-20.04 (release)	6m 45s
Check ubuntu-20.04 (devel)	5m 49s
Check ubuntu-20.04 (oldrel-1)	6m 15s
Check ubuntu-20.04 (oldrel-2)	6m 23s
Check ubuntu-20.04 (oldrel-3)	6m 33s
Check ubuntu-20.04 (oldrel-4)	5m 42s
	1h 30m 33s

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