docs: fix benchmark examples for Python 3.10

[git-jxj]

The multiple-choice adapter example in the English and Chinese benchmark guides raises SyntaxError on Python 3.10 and 3.11 because its f-string expression contains a newline escape.

Use string concatenation for the options heading and joined options so the example works on the supported Python versions and produces the same prompt text.

Validation: both complete snippets compile on Python 3.10. The extracted formatter was exercised with real Sample objects across four cases per language, including empty and Unicode options, and matches the previous output on Python 3.13. make lint and the CI lite smoke test with the mock model pass.