Java link goes to JRE

OpenWhisk requires the JDK, the link that sends you to Java goes to the JRE, it's not the clearest on how to get to the right place.

## Steps to reproduce the issue:

1.   Go to https://github.com/apache/openwhisk#quick-start
2.   Click the Java link
3.   Observe this goes to the JRE page, and it can be a little hard to get to the JDK page.