gmagick: remove useless wrapping para tag around &warn.experimental; (#5133)

Author: Girgias

reference/gmagick/book.xml

@@ -9,8 +9,8 @@
 
  <preface xml:id="intro.gmagick">
   &reftitle.intro;
+  &warn.experimental;
   <para>
-   &warn.experimental;
    Gmagick is a php extension to create, modify and obtain meta information of images using the GraphicsMagick API. 
   </para>
   <para>