Merge bitcoin#19344: docs: update testgen usage example

e12e970 docs: match usage text to script and location (Peter Bushnell)

Pull request description:

  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.

ACKs for top commit:
  fanquake:
    ACK e12e970 - this looks correct.

Tree-SHA512: b7ab61e19a54597a8fbd1844b9cfaef78879e53b882eefe4e0140fa115674df7f061e468835186963b89c963244a17d922f2ad0829b10f62b84f02019ee33edb

contrib/testgen/README.md

@@ -4,5 +4,5 @@ Utilities to generate test vectors for the data-driven Bitcoin tests.
 
 Usage:
 
-    PYTHONPATH=../../test/functional/test_framework ./gen_key_io_test_vectors.py valid 50 > ../../src/test/data/key_io_keys_valid.json
-    PYTHONPATH=../../test/functional/test_framework ./gen_key_io_test_vectors.py invalid 50 > ../../src/test/data/key_io_keys_invalid.json
+    PYTHONPATH=../../test/functional/test_framework ./gen_key_io_test_vectors.py valid 50 > ../../src/test/data/key_io_valid.json
+    PYTHONPATH=../../test/functional/test_framework ./gen_key_io_test_vectors.py invalid 50 > ../../src/test/data/key_io_invalid.json