message: Minor fix on AutoGraph limitation doc
diff: @@ -284,7 +284,7 @@ A special case of hidden side effects are methods, which are commonly used
  to change the value of objects:
  ```
  -def MyClass(object):
  +class MyClass(object):
  def change(self):
  self.y += 1
  @@ -308,7 +308,7 @@ temporary objects when executing eagerly, but their number is greatly reduced
  in `@tf.function`:
  ```
  -def MyClass(object):
  +class MyClass(object):
  def change(self):
  self.y += 1
  return self
  