fixed typo in README.rst

[Opimenov]

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[googlebot]

Thanks for your pull request. t looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

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What to do if you already signed the CLA

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• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

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• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[codecov-io]

Codecov Report

Merging #27 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #27 +/- ## ======================================= Coverage 94.28% 94.28% ======================================= Files 16 16 Lines 525 525 Branches 20 20 ======================================= Hits 495 495 Misses 12 12 Partials 18 18

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[Opimenov]

I signed it!

[googlebot]

CLAs look good, thanks!