Update quickstart.md

[mrezabidhendi]

The Variable ABI was defined in all CAPS, but it was used in the function call in all lowercase which lead to error.

Description

Please include a summary of the changes and be sure to follow our Contribution Guidelines.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [ ] I have selected the correct base branch.
• [x] I have performed a self-review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [ ] Any dependent changes have been merged and published in downstream modules.
• [ ] I ran npm run lint with success and extended the tests and types if necessary.
• [ ] I ran npm run test:unit with success.
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• [ ] I ran npm run build and tested dist/web3.min.js in a browser.
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• [ ] I have updated the CHANGELOG.md file in the root folder.
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[danforbes]

Thank you for catching this! I think it would actually be better to change the variable name to lowercase.

[jdevcs]

@SantiagoDevRel changes mentioned in this PR should be merged via new PR created by you.