docs: scheduler: completion: Fix minor error in pseudo-code

Add missing address-of (&) operator in pseudo-code.

Signed-off-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230706114057.1120335-1-rick.wertenbroek@gmail.com