Fix typos in Potential docstring

[chriswmann]

What is this PR about?
A recent update to the Potential function's code examples included minor typos in model.py. Specifically, minus signs were incorrectly included in the description of the constraints for the first two examples in two locations. Additionally, the warnings section of the docstring was too verbose. See #6574 for details.

I've simply removed the minus signs where they were incorrectly included, so that the examples are correct and consistent.
I've also shortened the warning to be more succinct.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

N/A

New features

N/A

Bugfixes

N/A

Documentation

• Remove spurious minus signs from the first two code examples in the Potential function
• Shorten warning about applicability of Potential only to logp-based sampling

[codecov]

Codecov Report

Merging #6575 (908ef44) into main (534a9ae) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6575   +/-   ##
=======================================
  Coverage   92.01%   92.01%           
=======================================
  Files          91       91           
  Lines       15108    15108           
=======================================
  Hits        13901    13901           
  Misses       1207     1207           

Impacted Files	Coverage Δ
pymc/model.py	89.52% <ø> (ø)