doc: bump libvala dependency

[kaixoo]

OS 8 doesn't seem to have libvala-0.48-dev anymore

user@pc:~$ sudo apt install libvala-0.48-dev
Lecture des listes de paquets... Fait
Construction de l'arbre des dépendances... Fait
Lecture des informations d'état... Fait      
E: Impossible de trouver le paquet libvala-0.48-dev
E: Impossible de trouver de paquet correspondant à l'expression rationnelle « libvala-0.48-dev »

[jeremypw]

Should be OK as OS7 has v0.56. The libvala dependency automatically adjusts to the version of vala compiler installed so we should probably specify the same minimum version for valac

[kaixoo]

so we should probably specify the same minimum version for valac

I didn't want to touch the minimum version. Just adjusting the recommended version because 0.48 no longer exists in OS 8.

[jeremypw]

I don't know what the true minimum version of vala required to compile the code is now tbh. But we only support compiling on the current release of elementary (8) and the previous one (7) anyway so we generally bump the dependencies accordingly.