Fix iam_role_additional_policies

[SamuZad]

What does this PR do?

Inspired by a detailed bug report, I implemented the proposed change to iam_role_additional_policies.

• The type of the variable is changed
• The conditional evaluating whether it is necessary to use the variable has been updated
• The documentation has been updated to reflect the change
• The example was updated to show iam_role_additional_policies in action
• Fixed an extra typo regarding subnets in the example

Motivation

A fix for #30

More

• Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• Yes, I ran pre-commit run -a with this PR

For Moderators

• E2E Test successfully complete before merge?

Additional Notes

[vara-bonthu]

@SamuZad Thanks for the PR 🚀 LGTM 👍🏼