theming guide URL is broken

[bumbapartha]

Bug, feature request, or proposal: Bug

What is the expected behavior?

Include the core and theme styles contains a link with text 'theming guide' which should take user to a proper 'theming guide' page.

What is the current behavior?

This link is now taking user to https://github.com/angular/material2/blob/master/guides/guides/theming.md which is a broken link.

What are the steps to reproduce?

Click on theming guide link.

[devversion]

We already have a PR for it. See #2400. And this issue is a duplicate of #2461

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