Incorrect function names in Object class

[zwade]

The documentation currently lists the Object class as having two methods, string2typeand type2string. However, as of nodegit@0.22.1, these functions do not exist and are instead called string2Type and type2String.

[RaevLogic]

I've taken care of this on my local build. I will submit a PR when I get a chance either tonight or tomorrow morning.