📚 Fix `SequenceSet#cover?` documentation #1804

Job	Run time
ruby-versions / Generate Ruby versions	3s
build (3.1 / ubuntu-latest)	41s
build (3.1 / macos-latest)	32s
build (3.1 / windows-latest)	1m 29s
build (3.2 / ubuntu-latest)	47s
build (3.2 / macos-latest)	39s
build (3.2 / windows-latest)	2m 14s
build (3.3 / ubuntu-latest)	43s
build (3.3 / macos-latest)	29s
build (3.3 / windows-latest)	3m 15s
build (3.4 / ubuntu-latest)	39s
build (3.4 / macos-latest)	29s
build (3.4 / windows-latest)	3m 12s
build (head / ubuntu-latest)	40s
build (head / macos-latest)	31s
build (head / windows-latest)	2m 4s
	18m 27s

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