Fix a variable name that cause elm-live to crash in several circumstances

[lucamug]

This is causing elm-live to crash with 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 /Users/lucamug/p/elm-live/node_modules/crocks/Async/index.js:54:10
    at Object.fork (/Users/lucamug/p/elm-live/node_modules/crocks/Async/index.js:155:20)
    at /Users/lucamug/p/elm-live/node_modules/crocks/Async/index.js:254:41
    at settle (/Users/lucamug/p/elm-live/node_modules/crocks/Async/index.js:151:16)
    at ReadFileContext.callback (/Users/lucamug/p/elm-live/node_modules/crocks/Async/index.js:56:47)
    at FSReqCallback.readFileAfterOpen [as oncomplete] (fs.js:261:13) {
  code: 'ERR_INVALID_ARG_TYPE'
}```

[netlify]

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Review with commit f74f9d0

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