S3- Inconsistent/incorrect config key

[sbocinec]

cloud-nuke documentation states that all the config file config key starts with a capital letter, including s3 resource type, which config key should be S3.

Though, for s3 resource type, only lowercase config key s3 is a valid config option.

To make it clear, I suggest to rename s3 config key to S3 to be consistent with the other config keys. Changing the key, is a major breaking change though. Until then, I suggest to at least fix the doc (I'm going to ship a PR to address this)