Fix examples after removing deprecated Plot (see d804fd2)

e.g.
```
$ python examples/advanced/gibbs_phenomenon.py
Traceback (most recent call last):
  File "examples/advanced/gibbs_phenomenon.py", line 16, in <module>
    from sympy import var, sqrt, integrate, conjugate, seterr, Abs,
pprint, I, pi,\
ImportError: cannot import name Plot
```