fix: update stale merge-before-opened rate in README caveat #65

Open permadeath.com
targeting main from fix-stale-doc-figures
Open opened by and assigned to permadeath.com
targeting main from fix-stale-doc-figures

The README's caveat said 18%, but the generated report (stats.json's othNegativePct, and the method block in embed.html) puts the figure at 19.4%. The README was never updated after the underlying number changed.

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