docs: configure Pi-Hole v6 API with URL that ends with admin

[eoleedi]

Description

Illustrate a way to set up PiHole v6 smart card with a URL that still ends with /admin suffix

Fixes #924

Type of change

• 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)

Checklist:

• I've read & comply with the contributing guidelines
• I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• I have made corresponding changes to the documentation (README.md).
• I've checked my modifications for any breaking changes, especially in the config.yml file

[bastienwirtz]

Goood catch @eoleedi, thanks!