Fix README typo

[whereswaldon]

What does this do / why do we need it?

Just fixes a simple grammar mistake in the README, no big deal.

What should your reviewer look out for in this PR?

N/A

Do you need help or clarification on anything?

N/A

Which issue(s) does this PR fix?

N/A

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[whereswaldon]

I signed it
(can't tell whether automation has picked up on that or not)

[googlebot]

CLAs look good, thanks!

[sdboyer]

thanks!