docs(using_deis/using-buildpacks): fixing unexisting opdemand/example-ruby-sinatra git repository

Git repository opdemand/example-ruby-sinatra does not exist. Fixed it by changing it to deis/example-ruby-sinatra
Also changed opdemand/example-java-jetty to deis/example-java-jetty
Merge pull request #1200 from mvanduijker/master

docs(using_deis/using-buildpacks): fixing unexisting example example-ruby-sinatra git repository