Updated with correct syntax to access default_tags variable.

[roncrivera]

While running terraform apply --var-file="credentials.tfvars", the following errors are being returned:

Error: output.default_tags: invalid variable syntax: "default_tags". Did you mean 'var.default_tags'? If this is part of inline `template` parameter
then you must escape the interpolation with two dollar signs. For
example: ${a} becomes $${a}.

Error: module.aws-vpc.output.default_tags: invalid variable syntax: "default_tags". Did you mean 'var.default_tags'? If this is part of inline `template` parameter
then you must escape the interpolation with two dollar signs. For
example: ${a} becomes $${a}.

This appears to be due to the incorrect access to the default_tags variable.

Applying the changes in this PR fixes the issue.

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[rsmitty]

lgtm. won't get checked by CI. merging.