Outdated Airflow create_user command in instructions (update for instructions)

Title: Outdated Airflow create_user command in instructions

Description: The provided instructions for creating a user in Airflow use the old create_user command syntax. However, in the newer version of Airflow (Airflow 2), the correct command is users create.

Details: The current documentation instructs users to utilize the following command:

airflow create_user \
  --role Admin \
  --username $USER_FIRST \
  --firstname $USER_FIRST \
  --lastname $USER_LAST \
  --email $EMAIL \
  --password $PASSWORD

This command is outdated and is not supported in Airflow 2.

Expected Behavior: Instructions should utilize the updated command syntax compatible with Airflow 2:

airflow users create \
    --role Admin \
    --username $USER_FIRST \
    --firstname $USER_FIRST \
    --lastname $USER_LAST \
    --email $EMAIL \
    --password $PASSWORD

Actual Behavior: Using the outdated command results in an error or unrecognized command in the Airflow 2 environment.

Steps to Reproduce:

1. Install Airflow 2.
2. Attempt to create a user using the provided create_user command.
3. Observe the error indicating the command is not recognized.

Suggested Fix: Update the documentation to use the correct command syntax for creating a user in Airflow 2.