stackinit: Fix comment for test_small_end

In union test_small_end, the small members are three and four.

Fixes: e71a29db ("stackinit: Add union initialization to selftests")
Closes: https://lore.kernel.org/CAMuHMdWvcKOc6v5o3-9-SqP_4oh5-GZQjZZb=-krhY=mVRED_Q@mail.gmail.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/3f8faa2d7d0d6b36571093ab0fb1fd5157abd7bb.1738593178.git.geert+renesas@glider.be


Signed-off-by: Kees Cook <kees@kernel.org>