x86/kconfig: Fix typo in the CONFIG_CMDLINE_BOOL help text

Signed-off-by: Sébastien Hinderer <Sebastien.Hinderer@ens-lyon.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Samuel Thibault <Samuel.Thibault@ens-lyon.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>