fetch-assets copies a non-existing directory from material2-docs-content

[jpmckearin]

I am not sure if the issue concerns this repo or the other one. angular/material2-docs-content repo no longer contains the overview directory that is specified in tools/fetch-assets.sh

[amcdnl]

Looks like its here to me: https://github.com/angular/material2-docs-content/tree/master/overview