message: correct code snippets to python3 style
diff: @@ -265,7 +265,7 @@ example:
  ```python
  constant = tf.constant([1, 2, 3])
  tensor = constant * constant
  -print tensor.eval()
  +print(tensor.eval())
  ```
  The `eval` method only works when a default `tf.Session` is active (see
  @@ -306,7 +306,7 @@ Note that you rarely want to use the following pattern when printing a
  ``` python
  t = <<some tensorflow operation>>
  -print t # This will print the symbolic tensor when the graph is being built.
  +print(t) # This will print the symbolic tensor when the graph is being built.
  # This tensor does not have a value in this context.
  ```
  