programs.zsh: rename oh-my-zsh to ohMyZsh

[Ma27]

Motivation for this change

This is intended to provide better consistency with other NixOS modules.
Please refer to mayflower#21 for further information. (/cc @WilliButz @fpletz)

I checked this change using the following configuration:

{
  # ...
  programs.zsh = {
    enable = true;

    ohMyZsh = {
      enable = true;
      theme = "norm";
    };
  };
}

I booted the VM using the following command:

NIXOS_CONFIG=`pwd`/vmtest.nix nixos-rebuild -I nixpkgs=$HOME/Projects/nixpkgs/ build-vm

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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