Forward exercise reference

Doc/Sd1/arrays.xml

@@ -1359,6 +1359,9 @@ values newArray  | 1| 2| F| 7| 9|  | ...</programlisting>
 ...
 		}
 	}</programlisting>
+
+            <para>You'll find a more sophisticated approach using interfaces
+            in upcoming <xref linkend="sd1SectPlotterInterface"/>.</para>
           </answer>
         </qandaentry>
       </qandadiv>

Doc/Sd1/interfacesAbstractClasses.xml

@@ -111,7 +111,7 @@ public void testApp() {
   </section>
 
   <section xml:id="sd1SectPlotterInterface">
-    <title>A plotter based on an interface</title>
+    <title>An interface based plotter</title>
 
     <qandaset defaultlabel="qanda" xml:id="sd1QandaPlotInterface">
       <qandadiv>