fix(docs): Error in running airflow locally

[buggythepirate]

There is an Error in the "Running airflow locally" documentation:

As described in the official airflow documentation, the doc is missing the airflow-init statement. The result is that the container won´t start properly as the initial database tables and the initial user are missing.

The fix is to add the missing line to the documentation and rearrange the paragraphs.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

[shirshanka]

Thanks for fixing the docs! This will probably help many people :)