i deleted the app about download in the dock, how can i restore it?

That's not an application; it is a shortcut to a document. Choose Home from the Finder's Go menu, open the Downloads folder, and drag it(or the Downloads folder itself) to the right of the Dock's divider line.

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It's also possible the OP deleted the Downloads folder itself. If so, just open Finder/Home and drag the Downloads folder to your Dock where it was before. This puts that folder link back down there.

If this is what you did, you need to actually click that folder and then when you list pops up of all the things contained in it, THEN you grab whatever file you want to delete and drag that to the trash.