docs: deployment -> custom env variables, env-cmd example is not working

Created by: VMois

Is this a bug report?

Incorrect information in docs -> Deployment.

Steps to Reproduce

1. Go to https://facebook.github.io/create-react-app/docs/deployment#customizing-environment-variables-for-arbitrary-build-environments
2. Follow the instruction and try to run env-cmd as suggested in docs. You will receive different error like FileNotFound etc. (I am using env-cmd version 9.0.1)

Expected Behavior

After some time, I found a solution. Working example:

"scripts": {
    "build:local": "env-cmd -f ./.env.production.local npm run build",
    "build:production": "env-cmd -f ./.env.production npm run build"
  }

Actual Behavior

In docs you can find something like this:

{
  "scripts": {
    "build:staging": "env-cmd .env.staging npm run build"
  }
}