Update readme with default openjdk version

[jlosito]

Description

The readme states that the default openjdk version that gets installed is 6, but when I run the default recipe without changing any attributes version 8 gets installed.

         * apt_package[openjdk-8-jdk, openjdk-8-jre-headless] action install
           - install version 8u232-b09-0ubuntu1 of package openjdk-8-jdk
           - install version 8u232-b09-0ubuntu1 of package openjdk-8-jre-headless

Check List

• [ ] All tests pass. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/TESTING.MD
• [ ] New functionality includes testing.
• [ ] New functionality has been documented in the README if applicable

[tas50]

Thanks @jlosito