nixos: Fix build of manual

Regression introduced by 520a43ced38843d5116594b359854e02fa7ed057.

Using XML tag characters for things that are not tags needs to be
properly indicated by an entity.

Signed-off-by: aszlig <[e-mail]>

nixos/modules/programs/oblogout.nix

···
         description = ''
           Icon theme for the buttons, must be in the themes folder of
           the package, or in
-          <filename>~/.themes/<name>/oblogout/</filename>.
+          <filename>~/.themes/&lt;name&gt;/oblogout/</filename>.
         '';
       };