fix: etc/litestream.yml - Corrected indentation for replica entries

[miyataka]

Description

fixed indentation and formatting based on https://litestream.io/guides/s3/

Motivation and Context

the document includes conflicting statements.

s3 configuration page says

dbs:
  - path: /path/to/local/db
    replica:
      url: s3://BUCKETNAME/PATHNAME

but etc/litestream.yml and Running in a Docker container page say

# dbs:
#  - path: /path/to/primary/db            # Database to replicate from
#    replicas:
#      - path: /path/to/replica           # File-based replication
#      - url:  s3://my.bucket.com/db      # S3-based replication

These are conflicting about replica and replicas key, and its child structure.

How Has This Been Tested?

Latest version (v0.3.13) runs with former one, but latter failed. on macOS.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)