docs: Fix default gcpkms region

[ethanal]

I believe this is an unintentional copy/paste from the AWS KMS seal docs, since us-east-1 is not a valid GCP region. I tried omitting the region and got the following error on startup:

Error parsing Seal configuration: 'region' not found for GCP CKMS wrapper configuration

This looks like it comes from here where I do not see a default region. Therefore, it looks like the region field is required.

[hashicorp-cla]

All committers have signed the CLA.

[VioletHynes]

Looks great! Thanks for the submission, I'm going to try and get this merged.