Correction to the /meta API documentation to include git field in output.

This is a change to the documentation for the [Meta API](http://developer.github.com/v3/meta/) so that the API docs reflect what I actually see when I do this:

```
$ curl https://api.github.com/meta
{
  "hooks": [
    "207.97.227.253/32",
    "50.57.128.197/32",
    "108.171.174.178/32",
    "50.57.231.61/32",
    "204.232.175.64/27",
    "192.30.252.0/22"
  ],
  "git": [
    "207.97.227.239/32"
  ]
}
```