add 'hexadecimal' keyword to .desktop file

otherwise GHex won't come up as a result when searching for 'hexadecimal' in gnome-software.

This was in Fedora 39, but should happen in any distribution.

Flathub.org do shows GHex when typing 'hexadecimal' because it searches inside the Description but gnome-software seems to only look in the Name, Summary and Keywords

Btw thank you for GHex, it has helped me tweak PDF files when working in Poppler issues.

Happy Christmas day! 🎄 🎅