Fix overloading example from the docs

[Czar102]

What's your issue about?

The docs give an example of an overloaded function:

@external
def add_seven(a: int128) -> int128:
    return a + 7

@external
def add_seven_with_overloading(a: uint256, b: uint256 = 3):
    return a + b

And add_seven_with_overloading(10) returns 13 instead of 17. How did this happen?

How can it be fixed?

Set the default value of b to seven instead of three. I think.