update npm start command

[rayng86]

npm script is broken and gives an error when you try running it on you local environment.

Error Message when running npm start:

Error: Cannot find module 'webpack-cli/bin/config-yargs'
Require stack:
../react-a11y-announcer/node_modules/webpack-dev-server/bin/webpack-dev-server.js

It looks like we are on "webpack-cli": "^4.1.0",, which now requires an updated command for npm start to work properly.

The fix was to change the following in package.json:

"scripts": { "start": "webpack-dev-server" }

to

"scripts": { "start": "webpack serve" }

[think-john]

Dear @brendanmckeown , I added you as a reviewer because Raymond here (a new member of the Comcast Corporate team) mentioned that he had some observations about this repo, and I told him "by all means, submit a pull request." So here is that pull request!

Do you think there is someone else that should get this PR?