Add s3:HeadBucket as a secret

In order to prevent AWS employees from using the non-existent IAM privilege `s3:HeadBucket`, that string could be viewed as a secret, which maybe would cause an alert every time someone tries to use that in the AWS docs. For example, this doc just appeared: https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_evaluation-logic-cross-account.html

That privilege does not exist. AWS should stop mentioning it in their docs.

The text pattern could be added at https://github.com/awslabs/git-secrets/blob/8450ad79fdb31fb685cd38a1bb8a62b3788a9858/git-secrets#L238

Start by reading the pattern list around line 238 in git-secrets and compare the requested s3:HeadBucket string with existing AWS secret patterns. Done means the exact privilege is covered by the scanner's pattern set and the project's existing validation passes.

Written by the indexing model from the issue text.