Fix typos in code comments

[mokashang]

Description

Fix three small typos in code comments:

• pgcli/pgexecute.py: funcion → function, tem → them
• tests/test_pgexecute.py: relly → really

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected (this may include manually testing them in your local environment, or in other available environments). Cross this out if not relevant (for example, if you're making a documentation change).
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)

[dbaty]

Let's not pollute the changelog with this: it's not useful to users. Can you remove this, please?

Also, I am not sure your changes warrant an update of AUTHORS. Unless you absolutely want your name there, of course. ;)