Small phrasing fix

[rafadaguiar]

In /stacks_queues/sort_stack/sort_stack_solution.ipynb there's a small phrasing issue on the line:
"While buffer is empty or buffer top is > than temp\n".

It should be "While buffer is not empty or buffer top is > than temp\n" as implied in the code below:

class MyStack(Stack):

    def sort(self):
        buff = MyStack()
        while not self.is_empty():
            temp = self.pop()
            while not buff.is_empty() and buff.peek() > temp:
                self.push(buff.pop())
            buff.push(temp)
        return buff

[donnemartin]

Hi Rafael,

Good catch, would you like to submit a PR? Otherwise I'm happy to fix it.

-Donne