DOC: Update README.md with correct function names

[nijanthanvijayakumar]

Quinn version checks

• [X] I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

README.md

Documentation problem

The README.md file's Column Extensions section contains functions such as nullBetween, isFalsy etc., that have a different name in the functions.py file.

Suggested fix for documentation

Update the README.md file to reflect the correct function name and their description (matching with the doc-string).

[fatemetardasti96]

I would like to work on this issue as my first contribution

[fatemetardasti96]

here is the PR: https://github.com/MrPowers/quinn/pull/240