Fix docs referencing outdated React Router <Link> API

[andrewagain]

The current docs reference this react-router page which is outdated. For instance it mentions the onlyActiveOnIndex prop which does not exist anymore in react-router.

[KyleAMathews]

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[KyleAMathews]

Ah nice! Yeah definitely needed updating.

Could we just link to their master branch though instead of a commit?

[andrewagain]

@KyleAMathews changed!

[KyleAMathews]

Perfect, thanks!

[jlengstorf]

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