JS: Fix undefined file reference when invoked from another directory

[robatron]

Fixes an undefined-file error when attempting to access the API when JavaScript server is started from a directory other than the project root directory, e.g.,

$ PORT=3001 node node_modules/react-tutorial/server.js

Server started: http://localhost:3001/
undefined:1
undefined
^

SyntaxError: Unexpected token u
    at Object.parse (native)
    at ReadFileContext.callback (/Users/rob/code/react-tutorial-official/server.js:28:19)
    at FSReqWrap.readFileAfterOpen [as oncomplete] (fs.js:303:13)

This change makes the comments.json file reference relative to the server.js script.

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[robatron]

Matches the script-relative path pattern used to reference the project's 'public' directory on line 23.

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[zpao]

Awesome, thanks!