fix typo in stop.py - options.max_poll -> options.max_polls

[ColemanTom]

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

I did not update any tests. The pathway leads to failure no matter what, this just fixes the exception type and message. No documentation updates are required.

[ColemanTom]

As a note, I'm kind of surprsied that no CI tools are catching this bug? Misnamed variables should be fairly apparent I would think?

[oliver-sanders]

As a note, I'm kind of surprsied that no CI tools are catching this bug? Misnamed variables should be fairly apparent I would think?

Codecov reports the line as covered, which is is, it's just that a different error gets raised to the one intended.

MyPy raises no concern as Options objects aren't introspectable in this way.