Update 13-iterators.md, openIterableDir not a method anymore.

[aboudekahil]

std.fs.cwd().openIterableDir doesn't compile in zig 0.13.0. Suggested an alternate code that compiles and returns an iterator over the directory.

[Sobeston]

Closed in favour of dc25911fd64684020d336a664f46d94d30618b97