Update delete-by-query.asciidoc (#22519)

Fixed some grammatical mistakes

docs/reference/docs/delete-by-query.asciidoc

@@ -294,7 +294,7 @@ GET /_tasks/taskId:1
 
 The advantage of this API is that it integrates with `wait_for_completion=false`
 to transparently return the status of completed tasks. If the task is completed
-and `wait_for_completion=false` was set on it them it'll come back with a
+and `wait_for_completion=false` was set on it then it'll come back with 
 `results` or an `error` field. The cost of this feature is the document that
 `wait_for_completion=false` creates at `.tasks/task/${taskId}`. It is up to
 you to delete that document.