[DOC] Minor fix on AutoGraph limitation doc

[kooyunmo]

There needs a minor correction for the AutoGraph limitation example.

def MyClass(object):
  def change(self):
    self.y += 1

c = MyClass()
while x > 0:
  c.change()  # Problem -- modification to c.y is not visible here!

and

def MyClass(object):
  def change(self):
    self.y += 1
    return self

c = MyClass()
while x > 0:
  c = c.change()  # Okay -- c is now a loop var.

In these examples, def MyClass should be class MyClass

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[gbaned]

@kooyunmo Can you please sign CLA. Thanks!

[kooyunmo]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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