Update vala.md with compile commands for gtk4

I read the introduction on the website but there was no clue on how to compile the program. Initially I just tried replacing gtk+-3.0 with gtk+-4.0 and it always failed. After a couple hours of searching I finally found the .vapi files! I was quite surprised to find the names were different I didn't find any hint for that in the official documentation.

Anyway I think it would help newcomers to have the compile command in the tutorial. At least it would have helped me...

Perhaps linking to a gtk4 tutorial would also be better I am pretty sure the linked tutorial do not compile under gtk4. I will work on porting the current one if I have the free time and can figure out how gtk4/vala works...