Update URL for EIA form 923
Update URL for EIA form 923

It looks like the EIA has changed their base URL from
`https://www.eia.gov/electricity/data/eia923/archive/xls` to
`https://www.eia.gov/electricity/data/eia923/xls` for all years
before 2017.

catalyst-cooperative/pudl,catalyst-cooperative/pudl