Update theme path

[peluprvi]

Bug, feature request, or proposal:

I think after #3776, core/theming/prebuilt path need to be changed to prebuilt-themes in some of those files.

I had to change my css path to :

<link rel="stylesheet" href="../node_modules/@angular/material/prebuilt-themes/indigo-pink.css">

[devversion]

• Only the paths in the markdown files need to be updated
• @jelbourn didn't want to update it yet, because it would just confuse (the new paths arent released yet)

Also the core/theming/prebuilt path is still valid inside of our demo-app or e2e-app. Just in releases this path will be invalid.

If necessary I can provide a PR that can be merged before we publish a new release.

[jelbourn]

Everything should be updated everywhere now.

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