Review: polars.md

[mmcky]

Part of #40 (Phase 1).

Native-speaker review of polars.md by @Honaminto.

French (read this)	lectures/polars.md
English source	lectures/polars.md

How to reply. Open a PR editing the French directly — one PR for this lecture, titled e.g. [polars] Corrections — or leave line comments / suggestion blocks on it. Priorities: meaning errors, wrong technical terms, inconsistent terminology, unnatural French (academic register), anything in code comments that looks wrong. Ignore spacing before : ; ! ?, errors in the English, and the translation: header. A wrong term that recurs belongs on the terminology issue, not here.

Done when a PR by @Honaminto touching this file is merged, or he states here that the lecture needs no change. Either outcome is recorded; nothing is ticked by hand.