nixos/manual: add environment.systemPackages option in an example

The environment.systemPackages option lacks in the example which is used in the next paragraph to explain merging of the options defined by multiple modules.

authored by Suwon Park and committed by GitHub 7948e678 4406e51a

+1
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nixos/doc/manual/configuration/modularity.xml
··· 36 { <xref linkend="opt-services.xserver.enable"/> = true; 37 <xref linkend="opt-services.xserver.displayManager.sddm.enable"/> = true; 38 <xref linkend="opt-services.xserver.desktopManager.plasma5.enable"/> = true; 39 } 40 </programlisting> 41 Note that both <filename>configuration.nix</filename> and
··· 36 { <xref linkend="opt-services.xserver.enable"/> = true; 37 <xref linkend="opt-services.xserver.displayManager.sddm.enable"/> = true; 38 <xref linkend="opt-services.xserver.desktopManager.plasma5.enable"/> = true; 39 + <xref linkend="opt-environment.systemPackages"/> = [ pkgs.vim ]; 40 } 41 </programlisting> 42 Note that both <filename>configuration.nix</filename> and