[Doc] Fix for UC on AWS guide - use databricks_aws_unity_catalog_assume_role_policy where necessary (#4109)

## Changes
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Resolves #3964

## Tests
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- [ ] `make test` run locally
- [x] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
- [ ] relevant acceptance tests are passing
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Author: alexott

docs/guides/unity-catalog.md

@@ -262,7 +262,7 @@ resource "aws_iam_policy" "external_data_access" {
 
 resource "aws_iam_role" "external_data_access" {
   name                = local.uc_iam_role
-  assume_role_policy  = data.aws_iam_policy_document.this.json
+  assume_role_policy  = data.databricks_aws_unity_catalog_assume_role_policy.this.json
   managed_policy_arns = [aws_iam_policy.external_data_access.arn]
   tags = merge(var.tags, {
     Name = "${local.prefix}-unity-catalog external access IAM role"