Read binary data in from_buffer documentation examples

[xiehuanyi]

The module usage example and from_buffer() docstring currently open the input file in text mode. On Python 3, a PDF header containing non-UTF-8 binary bytes raises UnicodeDecodeError before identification reaches libmagic.

Use binary mode and a context manager in both examples. Read the 2048 bytes recommended in the README and show the MIME result, whose output stays consistent across libmagic versions.

Validation on Python 3.12.15 with libmagic 5.45:

• Reproduced the original decoding error by adding a binary PDF header comment to the repository's otherwise ASCII PDF fixture.
• Executed both updated examples against that binary header and the original fixture: all four executions matched application/pdf and closed the file handle.
• Full project tests: 14 passed, 2 skipped.
• git diff --check passed.

Prepared by OpenAI Codex (GPT-6.1-sol).

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