Docs: Add missing full stop in `WP_Duotone` class method DocBlocks. #16824

Job	Run time
QUnit Tests / Run QUnit tests	2m 27s
Slack Notifications / Prepare notifications	4s
Slack Notifications / Success notifications	2s
Slack Notifications / Fixed notifications	0s
Slack Notifications / Failure notifications	0s
Slack Notifications / Cancelled notifications	0s
Failed workflow tasks	0s
	2m 33s

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