fix: don't require the S3_SERVICE variable and set default appropriately, fix readme formatting

Proposed changes

Fixes issue where the S3_SERVICE variable should be optional but was being enforced as required. I have tested this with and without the env var specified and against both traditional and "directory" style s3 buckets.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

[x] I have read the contributing guidelines.
[x] I have signed the F5 Contributor License Agreement (CLA).
[x] If applicable, I have added tests that prove my fix is effective or that my feature works.
[x] If applicable, I have checked that any relevant tests pass after adding my changes.
[x] I have updated any relevant documentation (e.g. README.md).

fixes https://github.com/nginxinc/nginx-s3-gateway/issues/247

nginxinc / nginx-s3-gateway

fix: don't require the S3_SERVICE variable and set default appropriately, fix readme formatting #248

Proposed changes

Checklist