neomutt: added missing sort options

[bbaserdem]

Description

The allowed entries for programs.neomutt.sort were missing the last prefix, as shown in the documentation, under the sort entry.

Fixes #6263

Checklist

• Change is backwards compatible.

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• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

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