gh-157724: Fix garbage collector documentation typos

[Michael-WhiteCapData]

Fixes #157724.

Correct three concrete documentation defects in InternalDocs/garbage_collector.md:

• render the fixed-number phrase as prose instead of an HTML-like tag
• correct prefech to prefetch
• correct the Ryzen 5 7600X L2 cache unit from GB to MB

This deliberately leaves the broader storage-complexity question in the issue for separate discussion.

AI assistance was used to inspect the repository guidance and prepare this focused documentation patch; I reviewed the source and diff in full.

Validation: git diff --check.

• Issue: Typos in garbage collector docs #157724

[python-cla-bot]

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[StanFromIreland]

Seeing as this is internal documentation, and 2/3 of these fixes don't really matter in practice, I think we can close this.

[StanFromIreland]

Apologies, I see there is more to this on the issue. Please don't open PRs for issues where someone else has already noted they will do so, as such I'm closing so that the original author can fix them all.