docs: match usage text to script and location
docs: match usage text to script and location

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

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.
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