Readd Java HelloWorld example for intellij tutorial

[zachgk]

Description

This PR adds back the java HelloWorld demo that was removed as part of #14213 to fix the intellij on java tutorial. It also fixes the reference to the new location in the tutorial and includes a fix to the dot call.
resolves #15204
@lanking520 @aaronmarkham

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[piyushghai]

@zachgk @lanking520 This url still gives a 404. ( https://github.com/apache/incubator-mxnet/blob/java-api/scala-package/mxnet-demo/java-demo/src/main/java/mxnet/HelloWorld.java)
This refers to a tree java-api which was deleted after that branch was merged into master.
The tree location needs to be changed to master