Fix: update type for Dex fee when constructing a checkpoint.

[0xKitsune]

Right now, when constructing a checkpoint, we are saving the Dex fee as a String, but it needs to be a number. This can be fixed very simply but just switching the existing code:

  dex_map.insert(
    String::from("fee"),
    format!("{:?}", uniswap_v2_dex.fee).into(),
);

to this:

  dex_map.insert(
    String::from("fee"),
     uniswap_v2_dex.fee.into(),
);

Link to the line of code