Fixing future warnings bug by switching from append to concat.

[simmonsj330]

Description

Fixed #1886 by changing from append to concat to get rid of future warnings. Tested with:
pytest tests/openbb_terminal/stocks/dark_pool_shorts/test_finra_model.py --rewrite-expected
This did not rewrite any files, so only modifying one file: finra_model.py

• Summary of the change / bug fix.
• Link # issue, if applicable.
• Screenshot of the feature or the bug before/after fix, if applicable.
• Relevant motivation and context.
• List any dependencies that are required for this change.

Others

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• My code passes all the checks pylint, flake8, black, ... To speed up development you should run pre-commit install.
• New and existing unit tests pass locally with my changes. You can test this locally using pytest tests/....