x86: bootparam.h: Delete duplicated word

Delete the repeated word "for".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200726004124.20618-2-rdunlap@infradead.org