docs: update fluentd formatter documentation link

[tykid16]

Summary

Updates the broken link to fluentd's formatter documentation in the README.

Changes

• Updated the formatter documentation URL from the old format
  (http://docs.fluentd.org/articles/formatter-plugin-overview) to the current format
  (https://docs.fluentd.org/formatter)
• Changed from HTTP to HTTPS for security

Context

The old URL returns a 404 error as fluentd has restructured their documentation. This PR updates the link to
point to the correct location in their current documentation structure.