fix(docs): fix url link to "Java Quickstart for ADK" page

[msampathkumar]

The link should be https://google.github.io/adk-docs/get-started/java/ instead of https://google.github.io/adk-docs/get-started/java.md

[koverholt]

Thanks for the catch! I added a note on the convention of using relative links to point to the MD file so we can fix both the Python and Java quickstart links here.

[koverholt]

Should be fixed now, thanks again!