fixes typo in readme.md

Hi Ryan,

Submitting a simple typo fix for you.  I mistakenly misread your README thinking I could call `.netx()` as a way to read the netmask from an instance.  Five minutes later, I realized this was just a simple typo for `.next()`.

I'm submitting a 2nd pull request with an override of `toString()` to return the netmask as a string.  Figured it would make your life easier to separate out this small fix into an easy pull request.

Cheers,
Steve