Update React Testing Library name (#2061)

Since the package was renamed to `@testing-library/react` we're trying to encourage people to drop writing "react-testing-library" in favor of "React Testing Library"

content/docs/addons-test-utils.md

@@ -19,7 +19,7 @@ var ReactTestUtils = require('react-dom/test-utils'); // ES5 with npm
 
 > Note:
 >
-> We recommend using [`react-testing-library`](https://git.io/react-testing-library) which is designed to enable and encourage writing tests that use your components as the end users do.
+> We recommend using [React Testing Library](https://testing-library.com/react) which is designed to enable and encourage writing tests that use your components as the end users do.
 >
 > Alternatively, Airbnb has released a testing utility called [Enzyme](https://airbnb.io/enzyme/), which makes it easy to assert, manipulate, and traverse your React Components' output.