Add missing ` from styleguide rule

[dustinsgoodman]

I was walking through the documentation and noticed a markdown issue. Cheers! 🍻

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[dustinsgoodman]

I signed it!

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