added the missing leading slash.md

[DarkenLight]

Description

Pointed out by @JamieMcH in this issue #769 (comment)

Fixes # (issue)

Describe the bug Example config.yml file has an incorrect string for background-image: that doesn't work for actually pointing to a local file.

Configuration The correct string should be:

background-image: "/assets/your/light/bg.png"

Current example misses the leading '/'

Type of change

• [ x] 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:

• [x ] I've read & comply with the contributing guidelines
• [ x] 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