[DOCS] Fix typo in task management API docs (#52867)

Author: nattachai305

docs/reference/cluster/tasks.asciidoc

@@ -223,7 +223,7 @@ GET _tasks?group_by=none
 
 The `X-Opaque-Id` header, when provided on the HTTP request header, is going to be returned as a header in the response as well as
 in the `headers` field for in the task information. This allows to track certain calls, or associate certain tasks with
-a the client that started them:
+the client that started them:
 
 [source,sh]
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