Fix mapnik for the example map

Replace the `<CssParaameter>`, which has been removed in mapnik 2+ cf: https://github.com/mapnik/mapnik/wiki/Mapnik2_Changes#cssparameters----properties

example-map/example.xml

@@ -2,9 +2,7 @@
 <Map bgcolor="#eeebe2" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over">
     <Style name="ocean">
         <Rule>
-            <PolygonSymbolizer>
-                <CssParameter name="fill">#b1d2f5</CssParameter>
-            </PolygonSymbolizer>
+            <PolygonSymbolizer fill="#b1d2f5" />
         </Rule>
     </Style>
     <Layer name="ocean" status="on" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">