docs: update example markup for fonts in README

[rupl]

Refs: CD-514, 25775b7, #448

Types of changes

• ✔️ Improvement (non-breaking change which iterates on an existing feature)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Security update (dependency updates, or to fix a vulnerability)

Description

Changes the README to show our recommended HTML for font-loading in case someone implements CD outside Drupal.

Steps to reproduce the problem or Steps to test

1. Just check the README and compare to a Drupal install.

Impact

README only. No change to any implementation.

PR Checklist

• I have followed the Conventional Commits guidelines.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.