Fix Go syntax in Leetcode 15: 3Sum two-pointer solution

[mosesbenjamin]

The current Go solution in this reference incorrectly uses a while loop, which is invalid in Go.

Incorrect:

while left < right {
    sum := nums[i] + nums[left] + nums[right]
    ...
}

The correct loop syntax to use in Go is:

for left < right {
    sum := nums[i] + nums[left] + nums[right]
    ...
}