Schedule tasks with `cron()` format need to follow AWS rules

[philsheard]

Context

I was deploying an update via Zappa and got an error with the ScheduleExpression.

botocore.exceptions.ClientError: An error occurred (ValidationException) when calling the PutRule operation: Parameter ScheduleExpression is not valid.

I hadn't made changes to the task schedule the Zappa settings so I guess Amazon might have changed the requirements in their backend.

Possible Fix

Some sort of check for validity in the settings would be good, but for now just a reference to these rules in the docs might help others.

Steps to Reproduce

1. Set a schedule task using standard cron syntax:

"events": [{
  "function": "myapp.callable",
  "expression": "cron(15 * * * ? *)"
}]

2. Deploy the branch
3. See error

Your Environment

• Zappa version used: 0.52.0
• Operating System and Python version: OSX, Python 3.8.6

[philsheard]

I've referenced the issue in a pull request with some suggested changes to the docs.