fix doc bug in cast OP, change 'float15' to 'float16' in support dtypes

raw doc: bool, float15, float32, float64, int8, int32, int64, uint8.
new doc: bool, float16, float32, float64, int8, int32, int64, uint8. 

python/paddle/fluid/layers/tensor.py

@@ -164,7 +164,7 @@ def cast(x, dtype):
         x(Variable): An input N-D Tensor with data type bool, float16,
             float32, float64, int32, int64, uint8.
         dtype(np.dtype|core.VarDesc.VarType|str): Data type of the output:
-            bool, float15, float32, float64, int8, int32, int64, uint8.
+            bool, float16, float32, float64, int8, int32, int64, uint8.
 
     Returns:
         Variable: A Tensor with the same shape as input's.