update dcap and dgrep urls

I started moving these examples to github. This updates the URL from the dartlang.org site.

@sethladd @kwalrath 
This URL doesn't work, due to the repo name being a bit different (sample-dgrep, not samples-grep). So the URL should be:

https://github.com/dart-lang/sample-dgrep/blob/master/README.md