doc: Subkernel exception fix

[architeuthidae]

ARTIQ Pull Request

Description of Changes

To match #2455 :)

Related Issue

Type of Changes

	Type
✓	:scroll: Docs

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• [x] Use correct spelling and grammar.

Documentation Changes

• [x] Check, test, and update the documentation in doc/. Build documentation (cd doc/manual/; make html) to ensure no errors.

Licensing

See copyright & licensing for more info. ARTIQ files that do not contain a license header are copyrighted by M-Labs Limited and are licensed under LGPLv3+.