Update the usage text in the README to match the usage text in the Python script.
https://github.com/bitcoin/bitcoin/blob/02b26ba1c119c7732f09f09e3b94f75effa569c0/contrib/testgen/gen_key_io_test_vectors.py#L9 https://github.com/bitcoin/bitcoin/blob/02b26ba1c119c7732f09f09e3b94f75effa569c0/contrib/testgen/gen_key_io_test_vectors.py#L10
Also to match the file names in the actual destination.
https://github.com/bitcoin/bitcoin/blob/02b26ba1c119c7732f09f09e3b94f75effa569c0/src/test/data/key_io_valid.json https://github.com/bitcoin/bitcoin/blob/02b26ba1c119c7732f09f09e3b94f75effa569c0/src/test/data/key_io_invalid.json
Following the README usage text generates new files when the user is likely to have wanted to update the existing files.