fix the pow doc bug (#1861)

Fix the pow doc bug, change the pow parameter from 2.0 to 3.0

doc/fluid/api_cn/layers_cn/pow_cn.rst

@@ -38,7 +38,7 @@ pow
     # example 2: argument factor is Variable
     factor_tensor = fluid.layers.fill_constant([1], "float32", 3.0)
     y_2 = fluid.layers.pow(x, factor=factor_tensor)
-    # y_2 is x^{2.0}
+    # y_2 is x^{3.0}