fix: use correct type for enable syscall var

[nschmeller]

Resolves LINK-1965

Summary

Customer getting this error:

│ Error: creating SSM Document (setup-lacework-agent): InvalidDocumentContent: Wrong default value for a "Boolean" parameters, got "false". at Line: 1, Column: 6081
│   with module.lacework_aws_ssm_agents_install.aws_ssm_document.setup_lacework_agent,
│   on .terraform/modules/lacework_aws_ssm_agents_install/main.tf line 1, in resource "aws_ssm_document" "setup_lacework_agent":
│    1: resource "aws_ssm_document" "setup_lacework_agent" {

How did you test this change?

I ran terraform apply, reproduced the error, then made this fix, tested again and it worked. It would be great to have an actual AWS backend in the CI pipeline

Issue

https://lacework.atlassian.net/browse/LINK-1965