x86: fix comment in protected mode header

Comments in arch/x86/boot/compressed/head_32.S erroneously refer to the
real mode pointer as the second and the heap area as the third argument
to decompress_kernel(). In fact, these have been the first and second
argument, respectively, since v2.6.20.

This patch corrects the comments. It introduces no code changes.

Signed-off-by: Philipp Kohlbecher <xt28@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>