Update code block format for `.env.local` in README.md for highlight parameter.

[lamtrinhdev]

What's changing?

Update code block format from bash to .env in README.md for highlight parameter in .env.local file.

Correct the code block for .env.local that we use .env that refer from https://github.com/github-linguist/linguist/blob/master/lib/linguist/languages.yml .

Dotenv:
  type: data
  color: "#e5d559"
  extensions:
  - ".env"
  filenames:
  - ".env"
  - ".env.ci"
  - ".env.dev"
  - ".env.development"
  - ".env.development.local"
  - ".env.example"
  - ".env.local"
  - ".env.prod"
  - ".env.production"
  - ".env.sample"
  - ".env.staging"
  - ".env.test"
  - ".env.testing"
  tm_scope: source.dotenv
  ace_mode: text
  language_id: 111148035

[lamtrinhdev]

Dear @ethanis , @begonaguereca ,

I have a minor update at README.md. Could you please help me to review it?

Thanks, Lam