nixos/manual: typo

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nixos
doc
manual
installation
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nixos/doc/manual/installation/installing.xml
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<command>systemctl stop network-manager</command>.</para></listitem>
<listitem><para>The NixOS installer doesn’t do any partitioning or
-
formatting yet, so you need to that yourself. Use the following
+
formatting yet, so you need to do that yourself. Use the following
commands:
<itemizedlist>