message: Fix typos
  * Fix typos
  This PR fixes some typos: tf.constant, tf.placeholder.
  * Made tuple to real complex type.
  fixed `tf.Variable((12.3, -4.85), tf.complex64)` to generate a rank
  0 tensor instead of 1, the same for another error.
diff: @@ -29,8 +29,8 @@ Some types of tensors are special, and these will be covered in other
  units of the Programmer's guide. The main ones are:
  * `tf.Variable`
  - * `tf.Constant`
  - * `tf.Placeholder`
  + * `tf.constant`
  + * `tf.placeholder`
  * `tf.SparseTensor`
  With the exception of `tf.Variable`, the value of a tensor is immutable, which
  @@ -64,7 +64,7 @@ The following snippet demonstrates creating a few rank 0 variables:
  mammal = tf.Variable("Elephant", tf.string)
  ignition = tf.Variable(451, tf.int16)
  floating = tf.Variable(3.14159265359, tf.float64)
  -its_complicated = tf.Variable((12.3, -4.85), tf.complex64)
  +its_complicated = tf.Variable(12.3 - 4.85j, tf.complex64)
  ```
  Note: A string is treated as a single item in TensorFlow, not as a sequence of
  @@ -79,7 +79,7 @@ initial value. For example:
  mystr = tf.Variable(["Hello"], tf.string)
  cool_numbers = tf.Variable([3.14159, 2.71828], tf.float32)
  first_primes = tf.Variable([2, 3, 5, 7, 11], tf.int32)
  -its_very_complicated = tf.Variable([(12.3, -4.85), (7.5, -6.23)], tf.complex64)
  +its_very_complicated = tf.Variable([12.3 - 4.85j, 7.5 - 6.23j], tf.complex64)
  ```
  @@ -275,8 +275,8 @@ Graphs and Sessions for more information).
  Sometimes it is not possible to evaluate a `tf.Tensor` with no context because
  its value might depend on dynamic information that is not available. For
  -example, tensors that depend on `Placeholder`s can't be evaluated without
  -providing a value for the `Placeholder`.
  +example, tensors that depend on `placeholder`s can't be evaluated without
  +providing a value for the `placeholder`.
  ``` python
  p = tf.placeholder(tf.float32)
  