x86/boot: Repair kernel-doc for boot_kstrtoul()

Adjust the kernel-doc comment to have the proper function name:
boot_kstrtoul().

  [ bp: Massage commit message. ]

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221031094835.15923-1-lukas.bulwahn@gmail.com