[Docs] Radio Overview References Incorrect Selector

[adammedford]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Docs for Radio button should refer to <md-radio-button> under the heading Radio-button label

What is the current behavior?

References <md-checkbox> instead

What are the steps to reproduce?

Viewable at:
https://material.angular.io/components/radio/overview

Offending file:
https://github.com/angular/material2/blob/1da2b70668160b2b3e38bc51537c46303114a7d1/src/lib/radio/radio.md

Would submit a PR but it is such a minor issue that I assume someone else will fix it while they are working on something else.

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