Fix environment file example

[developius]

Signed-off-by: Finnian Anderson get@finnian.io

In the README, there is an example for the environment_file which has the incorrect syntax for the yaml.

Description

This:

environment:
  - access_key: key1
  - secret_key: key2

Should be this:

environment:
  access_key: key1
  secret_key: key2

I've also added a couple of missing yaml code block syntax highlighting labels.

Motivation and Context

Misleading example, had to battle with it for 10 minutes so I thought it would be nice to update the example so others don't encounter the same issue.

• I have raised an issue to propose this change (required)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

Good find 👍