dbt Hamilton example update requirements and readme

[datarshreya]

PR for issue #240

Changes

1. Updated README.md in /examples/dbt/: Changed pip install - r requirements.txt to pip install -r requirements.txt
2. Updated requirements.txt in /examples/dbt/: Changed dbt to dbt-core, added requirement for pandera

How I tested this

dbt run before and after

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[elijahbenizzy]

LGTM, thanks!