rust: docs: add missing newline to printing macro examples

Fix adding a newline at the end of the usage of pr_info! in the
documentation

Fixes: e3c3d345 ("docs: rust: Add description of Rust documentation test as KUnit ones")
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1139


Signed-off-by: Alban Kurti <kurti@invicto.ai>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20250206-printing_fix-v3-1-a85273b501ae@invicto.ai


[ Replaced Closes with Link since it fixes part of the issue. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>