[swift-api-dump] Omit 'python' in usage (NFC)
[swift-api-dump] Omit 'python' in usage (NFC)

- Since this file has a Python shebang (`#!/usr/bin/env python`) at the top, it can be invoked directly, without specifying which Python interpreter to use on the command line.
- Use the `%(prog)s` substitution instead of writing the program name again explicitly.
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