docs: scheduler: completion: Update member of struct completion

The member "wait" in struct completion isn't of type wait_queue_head_t
anymore, as it is now "struct swait_queue_head", fix it to match with
the current implementation.
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240828165036.178011-1-richard120310@gmail.com

Documentation/scheduler/completion.rst

@@ -51,7 +51,7 @@ which has only two fields::
 
 	struct completion {
 		unsigned int done;
-		wait_queue_head_t wait;
+		struct swait_queue_head wait;
 	};
 
 This provides the ->wait waitqueue to place tasks on for waiting (if any), and