fix typo in readme

[tobowers]

encryption was misspelled in the known security flaws section.

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[tobowers]

I signed it!

[googlebot]

CLAs look good, thanks!

[devinlundberg]

Sorry there were two PRs in my inbox for this typo at the same time and I merged the later one first since it was on the top of my email. I'll keep an eye out for that in the future. Thanks for contributing!