[ppa-experiment] Inversion of meaning for ppa explainer

[intentionally-left-nil]

An opt-in approach might enable weaker privacy protections, but would not necessarily provide better data in exchange. Having more data means both better measurement accuracy and an ability to add more noise on a per-person basis, meaning better privacy.

Presumably this should read "An opt-in approach might enable stronger privacy protections"

https://github.com/mozilla/explainers/tree/main/ppa-experiment#opt-out

I think the argument is absolute garbage, but at least you might as well use the correct words so it can be refuted properly.

[philipwhiuk]

The line should also clarify that having more data provides better privacy for those in the cohort. It does not provide better privacy for those outside.