maintainer-list: Fix M0ustach3 maintainer entry

Two different PRs, two different issues.

The first PR, chronologically-merged-in, #426687 introduced a new
module, and while reviewers “reviewed” the `meta.maintainers` entry,
they did not notice the maintainer was not in the maintainers list file.

 - https://github.com/NixOS/nixpkgs/pull/426687#discussion_r2311053832

Then, in #437310, merged a few days later, another new module was added
with M0ustach3 as a maintainer. At the very least, an attempt was made
in de66c73a to add the maintainer to the
list, but an unfortunate typo (or two?) in the maintainers list entry
means the two intended usage of the new entry still don't work.

The issue here is not from the contributors. The reviewers and
committers could have done better, but it's not even truly their fault.
The issue is the meta information in NixOS modules was never checked. In
any way AFAICT. Otherwise this would have been caught immediately, and
not four months later.

Anyway, the intent clearly is that the entry should matche the GitHub
username.