Update Zig Warning Message To Zig 0.15.0

[sneekyfoxx]

The warning message states that Zig 0.14 is in development but it is actually the newest stable release and Zig 0.15 is the development branch.

[matm]

Now Zig 0.15 is stable. This is a nice book to learn Zig, can the content be updated? Thanks!

[georg95]

Should be fixed when this pr merged:
#324