Fix typo in parallel-task documentation

[stefreak]

Changes

Fixes a simple typo. The parallel-task concept docs referred to a "customer wrapper"; I believe it was supposed to mean "custom wrapper".

How I tested this

(nothing to test)

Notes

(no notes)

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

thanks!