x86/Kconfig: Fix spelling mistake "effectivness" -> "effectiveness"

The Kconfig text contains a spelling mistake, fix it.
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-janitors@vger.kernel.org
Link: http://lkml.kernel.org/r/20190416105751.18899-1-colin.king@canonical.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/Kconfig

@@ -1499,7 +1499,7 @@ config X86_CPA_STATISTICS
 	depends on DEBUG_FS
 	---help---
 	  Expose statistics about the Change Page Attribute mechanims, which
-	  helps to determine the effectivness of preserving large and huge
+	  helps to determine the effectiveness of preserving large and huge
 	  page mappings when mapping protections are changed.
 
 config ARCH_HAS_MEM_ENCRYPT