fix chunk evaluator name in metrics

Author: typhoonzero

python/paddle/fluid/metrics.py

@@ -169,7 +169,7 @@ def eval(self):
         return self.value / self.weight
 
 
-class ChunkEvalutor(MetricBase):
+class ChunkEvaluator(MetricBase):
     """
     Accumulate counter numbers output by chunk_eval from mini-batches and
     compute the precision recall and F1-score using the accumulated counter