qt.section.md: fix xml syntax

doc/languages-frameworks/qt.section.md

···
   version = "1.0";
 
   buildInputs = [ qtbase ];
-  nativeBuildInputs = [ wrapQtAppsHook ]; <co xml:id'qt-default-nix-co-2' />
+  nativeBuildInputs = [ wrapQtAppsHook ]; <co xml:id='qt-default-nix-co-2' />
 }
 </programlisting>