Use strconv.Atoi to parse numWant in tracker implementation

I think this is what the code is supposed to do. However, the current implementation will try to get `numWant` as a query parameter, instead of parsing it from the URL.

As an example, if the URL has `numWant=100` in it, line 133 will cause the code to look for a query parameter called `100`, instead of converting `100` to an int.