samples(python): Updated use of model.load instances

The load() function returns a new object, and as such does not use the one it is called on.
This commit updates the uses of model.load in this program so it will work as intended and not throw an error.

samples/python/letter_recog.py

@@ -41,7 +41,7 @@ class LetterStatModel(object):
     train_ratio = 0.5
 
     def load(self, fn):
-        self.model.load(fn)
+        self.model = self.model.load(fn)
     def save(self, fn):
         self.model.save(fn)