Fix unreadable example in wxDataViewValueAdjuster documentation

@code tag must be used to make the code readable and using @example
seems to be unnecessary and just results in the creation of an extra
"Examples" page in the documentation containing only this example and an
absolute path (apparently not affected by STRIP_FROM_PATH) to
interface/wx/dataview.h file itself in the generated documentation.

interface/wx/dataview.h

@@ -3875,7 +3875,7 @@ class wxDataViewEvent : public wxNotifyEvent
     alternative to implementing an entire wxDataViewCustomRenderer
     specialization.
 
-    @example
+    @code
     // Markup renderer that removes bgcolor attributes when in selection
     class DataViewMarkupRenderer : public wxDataViewTextRenderer
     {
@@ -3904,7 +3904,7 @@ class wxDataViewEvent : public wxNotifyEvent
             }
         };
     };
-    @endexample
+    @endcode
 
     @since 3.1.1