[docs] incorrect the example of mackerel_monitor.external

[sogaoh]

About below:
https://registry.terraform.io/providers/mackerelio-labs/mackerel/latest/docs/resources/monitor#external

I think there is a incorrect statement:

headers = { "name" : "Cache-Control", "value" : "no-cache" }

I'd like you to correct it:

headers = { Cache-Control = "no-cache" }

My Example:

• https://github.com/sogaoh/mackerel-practice/blob/master/terraform-provider/serverless-stns/30_Monitor.tf

[heleeen]

Thank you for the information 🙏