docs/resource/lambda_permission: Fix cloudwatch example
docs/resource/lambda_permission: Fix cloudwatch example

source_account isn't valid for a CloudWatch Lambda Permission and
prevents CloudWatch from invoking the function as decribed at the
bottom of https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/CWE_Troubleshooting.html#LAMfunctionNotInvoked

Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws