Fixes typo in README.md

[stepri]

This PR fixes a small typo in a code example in the README.md file.

oAuth2client > oAuth2Client (capital C)

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[JustinBeckwith]

Greetings @stepri, and thanks for the contribution! Once you sign the CLA, we'd be happy to merge this :)

[stepri]

I signed it!

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CLAs look good, thanks!