fix: Update map function to work in Terraform 0.15

[matthewrkrieger]

Description

Update call to deprecated map function to use tomap instead

Motivation and Context

Resolves #43

https://www.hashicorp.com/blog/deprecating-terraform-0-11-support-in-terraform-providers

Breaking Changes

Will break Terraform <0.12

How Has This Been Tested?

• I have tested and validated these changes using the examples/complete project

$ terraform version
Terraform v0.15.0
on darwin_amd64
+ provider registry.terraform.io/hashicorp/aws v3.38.0

[antonbabenko]

@bryantbiggs Do you know what is going on with failed GH Actions / terraform_docs?

[bryantbiggs]

@antonbabenko it looks to be working as expected #45 - the only thing that looks slightly different here is that @matthewrkrieger is a first time contributor and Github doesn't automatically run actions for first time contributors without approval first

[antonbabenko]

@bryantbiggs https://github.com/terraform-aws-modules/terraform-aws-transit-gateway/runs/2511351637 - this one, it was for commit 7c7febb which failed.

[antonbabenko]

Thanks, @matthewrkrieger !

v2.1.0 has been just released.

[github-actions]

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