message: Update comments for variables in get_started
  This PR updates the comments in the Getting Started guide to make it more clear to which values they refer to.
diff: @@ -272,7 +272,7 @@ train = optimizer.minimize(loss)
  ```
  ```python
  -sess.run(init) # reset values to incorrect defaults.
  +sess.run(init) # reset variables to incorrect defaults.
  for i in range(1000):
  sess.run(train, {x: [1, 2, 3, 4], y: [0, -1, -2, -3]})
  @@ -317,7 +317,7 @@ y_train = [0, -1, -2, -3]
  # training loop
  init = tf.global_variables_initializer()
  sess = tf.Session()
  -sess.run(init) # reset values to wrong
  +sess.run(init) # initialize variables with incorrect defaults.
  for i in range(1000):
  sess.run(train, {x: x_train, y: y_train})
  