Fix navigation links in documentation.

[prsevero]

When navigating in the docs, these two links are broken.

The "Continue Reading" in http://facebook.github.io/react-native/docs/more-resources.html page, and "Previous" in http://facebook.github.io/react-native/docs/platform-specific-code.html.

This PR is to fix this.

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need the corporate CLA signed.

If you have received this in error or have any questions, please contact us at [email protected]. Thanks!

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!

[facebook-github-bot]

@hramos is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.