Fix format_number reversing digits

Enum.reverse/1 was reversing each chunk's characters, scrambling the
displayed numbers (e.g. 58,363 displayed as 85,363).

lib/microwaveprop_web/live/backfill_live.ex

@@ -262,9 +262,9 @@ defmodule MicrowavepropWeb.BackfillLive do
     |> String.graphemes()
     |> Enum.reverse()
     |> Enum.chunk_every(3)
-    |> Enum.map_join(",", &Enum.reverse/1)
+    |> Enum.map_join(",", &Enum.join/1)
     |> String.reverse()
-    |> then(fn s -> String.replace(s, ~r/^,/, "") end)
+    |> String.replace(~r/^,/, "")
   end
 
   defp format_number(n) when is_float(n), do: format_number(round(n))