Use singular form for a modifier.

Submitted by:	alc
MFC after:	3 days

sys/amd64/amd64/fpu.c

@@ -141,7 +141,7 @@ CTASSERT(sizeof(struct savefpu_ymm) == 832);
 /*
  * This requirement is to make it easier for asm code to calculate
  * offset of the fpu save area from the pcb address. FPU save area
- * must be 64-bytes aligned.
+ * must be 64-byte aligned.
  */
 CTASSERT(sizeof(struct pcb) % XSAVE_AREA_ALIGN == 0);