sum_two sample solution parameter

For the sum_two_challenge notebook ([link](https://github.com/donnemartin/interactive-coding-challenges/blob/master/math_probability/sum_two/sum_two_challenge.ipynb)), the Solution class template only takes one parameter:

```
class Solution(object):

def sum_two(self, val):
# TODO: Implement me
pass
```

That should change to two parameters:

```
class Solution(object):

def sum_two(self, a, b):
# TODO: Implement me
pass
```

Otherwise a TypeError is raised with the message:

`TypeError: sum_two() takes 2 positional arguments but 3 were given`