Use proper SPDX identifier for license

[lstrojny]

Tools that parse license identifiers fail with the current identifier of the csv crate. E.g. cargo-cyclonedx:

[2023-01-17T14:43:55Z ERROR cargo_cyclonedx::generator] Package csv has an invalid license expression, trying lax parsing (Unlicense/MIT): Invalid SPDX expression: invalid character(s)

This PR fixes it.

[atouchet]

Note: This change is included in #214.