dummymbuf: Fix code style

[kp]

Approved.

I was going to suggest not capitalising 'Fix', but apparently I am wrong about what our convention is there. See https://freebsdfoundation.org/wp-content/uploads/2020/11/Writing-Commit-Messages.pdf where Ed explicitly recommends "Capitalize the first letter of the subject", and I'm not going to argue with Ed.

I might include a 'No functional change' in the commit message, just to be doubly explicit about that, but the first line already heavily implies it.