Updates to readme

[greenozon]

Minor ticket to actualize readme page

current readme states:

Set AWS provider's region to us-east-1 or sa-east-1 when using [modules/pricing](https://github.com/terraform-aws-modules/terraform-aws-pricing/tree/master/modules/pricing) because AWS Pricing service is only available in these regions.

1. there is no region as sa-east-1 as stated in the readme
   I believe this should be ap-south-1

2. current list of AWS regions that supports pricing querying is as follows:

▶ $ aws pricing describe-services --region us-east-1
▶ $ aws pricing describe-services --region eu-central-1
▶ $ aws pricing describe-services --region ap-south-1

it means +1 new region: eu-central-1

The idea is to update readme and make it up-to-date

thanks