Describe the bug
In the help output, the severity is shown as:

  -s, --severity strings                  severities of security issues to be displayed (UNKNOWN,LOW,MEDIUM,HIGH,CRITICAL) (default [UNKNOWN,LOW,MEDIUM,HIGH,CRITICAL])

Note: these values are comma delimited

To Reproduce
Steps to reproduce the behavior:

1. Run trivy aws -h
2. Copy the default value shown for the severity flag
3. Paste default severity value and modify it to exclude the "UNKNOWN,LOW,MEDIUM" string
   Example:

trivy aws -s [HIGH,CRITICAL] --account sandbox

5. See error:

FATAL	Fatal error	flag error: report flag error: unable to parse flag: invalid argument "[[HIGH CRITICAL]]" for "--severity" flag: must be one of ["UNKNOWN" "LOW" "MEDIUM" "HIGH" "CRITICAL"]

Note: these values are space delimited

Expected behavior
The example shown in the help document should be helpful for a new trivy aws user. Copy-and-paste is a helpful way for a new trivy aws user to get started with using, customizing parameters for how the tool will run.