Update modeladmin.md

Corrected many broken link-renderings. Also removed the "Related" link to [genericviews Module](http://silverstripe.org/generic-views-module) because it doesn't work, and I can't find it elswhere.
@JorisDebonnet it looks like @sminnee has already merged it. Could you do another pull request to revert that specific part about summary_fields back to what it was?