fix typo

[chungshan]

fix typo in README

[google-cla[bot]]

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[chungshan]

@googlebot I signed it!

[acassirer]

Hi @virtualclone,

Thank you very much for contributing to Reverb. I've merged your PR in https://github.com/deepmind/reverb/commit/6aa2ed0aeb014782d81371c7a242fd7d90712837.

Cheers,