nixos manual: point to NixOps' homepage, not Github

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nixos
doc
manual
installation
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nixos/doc/manual/installation/obtaining.xml
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<para>Using NixOps, the NixOS-based cloud deployment tool, which
allows you to provision VirtualBox and EC2 NixOS instances from
declarative specifications. Check out the <link
-
xlink:href="https://github.com/NixOS/nixops">NixOps
-
homepage</link> for details.</para>
+
xlink:href="https://nixos.org/nixops">NixOps homepage</link> for
+
details.</para>
</listitem>
</itemizedlist>