Reframed a sentence

[grohan47]

Description

The warning for the manual GRUB update commands has been reframed to emphasize referring to a distro's docs.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• 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)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (correct file and folder structure)
• The background image for theme is called background.png
• theme.txt has correct 'desktop-image: "background.png"' attribute
• I updated themes.json file (JSON database)