Modify the description for 3D Pooling of PoolLayer.

[Kongsea]

When using PoolLayer for 3D Pooling, we can use as follows:
tl.layers.PoolLayer(x, pool=tf.nn.max_pool3d)

So the old description "you can choose tf.nn.max_pool and tf.nn.avg_pool for 2D or tf.nn.max_pool3d() and tf.nn.avg_pool3d() for 3D." is not exactly precise.
We eliminate the brackets of "tf.nn.max_pool3d()" and "tf.nn.avg_pool3d()" to be precise.