Migration script fixes

[akx]

• removes the note about using pipx – the invocation is wrong, and finally, it doesn't work when corrected:

  $ pipx run hatch-jupyter-builder migrate .
  usage: /Users/akx/.local/pipx/.cache/c28885c1d0910f0/bin/python -m hatch_jupyter_builder.migrate [-h] target_dir
  /Users/akx/.local/pipx/.cache/c28885c1d0910f0/bin/python -m hatch_jupyter_builder.migrate: error: unrecognized arguments: .
  

• fixes a typo that causes Git to say fatal: pathspec '.flake' did not match any files

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