remove trailing comma in package.json

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Status Job ID Name Coverage
  Pages
failed #307
docker
pages

00:10

 
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failed
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npm ERR! missing script: doc
npm ERR!
npm ERR! If you need help, you may report this error at:
npm ERR! <https://github.com/npm/npm/issues>

npm ERR! Please include the following file with any support request:
npm ERR! /builds/JadeProListening/html-custom-elements/npm-debug.log
ERROR: Build failed: exit code 1