Change "uft-8" to "utf-8".

API is returning "uft-8" for it's encoding, typo for "utf-8".  This breaks httpie, and probably other things:

```
$ http GET http://api.godoc.org/importers/github.com/jmcvetta/restclient
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: application/json; charset=uft-8
Date: Fri, 23 Aug 2013 00:08:48 GMT
Server: nginx/1.1.19
Transfer-Encoding: chunked


http: error: LookupError: unknown encoding: uft-8
```