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 branch 'master' of https://github.com/mvanduijker/deis