path argument must be string error on npm start

[shimwell]

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Describe the bug npm start results in a Path argument must be a string error, I believe this is the same error as https://github.com/firebase/firebase-tools/issues/2112 which got solved with this merge https://github.com/firebase/firebase-tools/pull/2206

Also I spotted a tiny typo on line 90 of the Readme where config-example.js is spelt congig-example.js

Thanks for making the example by the way it is just what I have been looking for

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Desktop (please complete the following information):

• OS: Ubuntu 18.04
• Browser Firefox