Fix `url.rootpath` typo
Fix `url.rootpath` typo

When starting up elm-live, I got this error:
```
internal/fs/utils.js:581
    throw new ERR_INVALID_ARG_TYPE(propName, ['string', 'Buffer', 'URL'], path);
    ^

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string or an instance of Buffer or URL. Received undefined
    at readFile (fs.js:317:10)
    at /home/path/to/node_modules/crocks/Async/index.js:54:10
    at Object.fork (/home/path/to/node_modules/crocks/Async/index.js:155:20)
    at /home/path/to/node_modules/crocks/Async/index.js:254:41
    at settle (/home/path/to/node_modules/crocks/Async/index.js:151:16)
    at ReadFileContext.callback (/home/path/to/node_modules/crocks/Async/index.js:56:47)
    at FSReqCallback.readFileAfterOpen [as oncomplete] (fs.js:261:13) {
  code: 'ERR_INVALID_ARG_TYPE'
}
```

After findind where the call occurred and console logging the stringified `url` object I got this output:
```
{
    "rootPath": "public/index.html",
    "getPath": "public/index.html",
    "isRoot": true
}
```

It looks like `url.rootpath` is a typo so I fixed it.
tomekwi/elm-live,tomekwi/elm-live