Fix wrong XML fragment syntax highlighting in liblzma docs

Turn off syntax highlighting entirely because using ".xml" here results
in total garbage in output for some reason (while not using any specific
language uses C++ highlighting which is confusing).

docs/doxygen/mainpages/liblzma.h

@@ -124,7 +124,7 @@ inside the @c ClCompile tag as well as add @c liblzma.lib to the @c
 AdditionalDependencies tag under @c Link itself. The result should look like
 the following, assuming liblzma sources are in c:\\src\\liblzma:
 
-@code
+@code{.unparsed}
   <ItemDefinitionGroup>
     <ClCompile>
       ... any existing lines here ...