rust: init: update `stack_try_pin_init` examples

rust: init: update `stack_try_pin_init` examples

Change documentation imports to use `kernel::alloc::AllocError`,
because `KBox::new()` now returns that, instead of the `core`'s
`AllocError`.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Jimmy Ostler <jtostler1@gmail.com>
Link: https://lore.kernel.org/r/ec8badbe94c5e78f22315325a7f2ae96129d6a65.1734674670.git.jtostler1@gmail.com
[ Fixed formatting of imports (still unordered). Slightly reworded
  commit. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>