NixOS manual: Update link to list of AMIs

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nixos
doc
manual
installation
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nixos/doc/manual/installation/obtaining.xml
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<listitem>
<para>Using AMIs for Amazon’s EC2. To find one for your region
and instance type, please refer to the <link
-
xlink:href="https://github.com/NixOS/nixops/blob/master/nix/ec2-amis.nix">list
+
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/virtualisation/ec2-amis.nix">list
of most recent AMIs</link>.</para>
</listitem>
<listitem>