Changed 'JAVAHOME' environment variable to 'JAVA_HOME' in README.md.

[mjordan]

Proposed changes

1. Brief description of changes applied

Fixed typo in README.md ('JAVAHOME' should be 'JAVA_HOME')

2. Any documentation on how to configure, test
   n/a

3. Any possible limitations, side effects, etc
   n/a

4. Reference any other pull requests that might be related.
   n/a

Types of changes

What types of changes does your code introduce?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Feature modification (Fix or feature that will cause existing functionality to not work as expected)
• [x ] Documentation

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Read the CONTRIBUTING doc
• Maintained the existing coding style and format
• Added tests that prove my fix is effective or that my feature works
• Added necessary documentation (if appropriate)
• Rebase and squash my commits. Submit one commit only, on top of the master branch.

Reviewers: @tlipkis, @clairegriffin, ...

Thanks for your contribution.