Fixed broken links on the Tips for Java Developers page
Fixed broken links on the Tips for Java Developers page

Tips for Java Developers page has 7 broken links resulting in 404.
This is because it was pointing to the master branch that no longer exists.
Updated all links to use the new default branch main.
https://github.com/cloudfoundry/java-buildpack/issues/814
cloudfoundry/docs-buildpacks