[2.1.x] Fixed LayerMapping encoding in geodjango tutorial.

GDAL >= 1.9 automatically converts latin-1-encoded strings from Shapefiles to
UTF-8.
Backport of 388165ad from master.

docs/ref/contrib/gis/tutorial.txt

@@ -457,10 +457,7 @@ with the following code::
     )
 
     def run(verbose=True):
-        lm = LayerMapping(
-            WorldBorder, world_shp, world_mapping,
-            transform=False, encoding='iso-8859-1',
-        )
+        lm = LayerMapping(WorldBorder, world_shp, world_mapping, transform=False)
         lm.save(strict=True, verbose=verbose)
 
 A few notes about what's going on: