add example of s3 key (file path)

I suggest we add the example for key value (s3).

On my first attempt to run GET, I configured the key to have leading slash (I was thinking about usual form of full path) and faced the error:

$ terraform init
Initializing modules...
- gitlab_ref_arch_aws in ../../modules/gitlab_ref_arch_aws
...
...
...
- gitlab_ref_arch_aws.sidekiq in ../../modules/gitlab_aws_instance

Initializing the backend...

Error: key must not start with '/'

I assume that whoever did not work with terraform and s3 (and even if they did) will likely think of proper format of full path to be the usual one (starting with slash) and if they attempt to use such path will face the same error.

Now, it is true that we are linking to terraform s3 docs already and one might say that's enough, but maybe it's a good idea to note this part explicitly to avoid cases where people face this error on their first run. It's not the end of the world, but it might lead to better experience overall.