amd64/mp_machdep.c: remove double 'for' in comment

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days

sys/amd64/amd64/mp_machdep.c

@@ -280,7 +280,7 @@ init_secondary(void)
 	}
 
 	if (!fred) {
-		/* The DB# stack, used for for IDT, runs on IST4. */
+		/* The DB# stack, used for IDT, runs on IST4. */
 		np = ((struct nmi_pcpu *)&dbg_stack[DBG_STACK_SIZE]) - 1;
 		np->np_pcpu = (register_t)pc;
 		pc->pc_common_tss.tss_ist4 = (long)np;