factor: improve comments for sieving logic... also fix a typo

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/coreutils/factor.c b/coreutils/factor.c
index 5e23c6c..2817534 100644
--- a/coreutils/factor.c
+++ b/coreutils/factor.c
@@ -107,8 +107,8 @@
 		+ (MULTIPLE_OF_5 - 6 * SHIFT_5)
 		+ (MULTIPLE_OF_7 - 9 * SHIFT_7)
 		//+ (MULTIPLE_OF_11 - 15 * SHIFT_11)
-		//+ (MULTIPLE_OF_11 - 18 * SHIFT_13)
-		//+ (MULTIPLE_OF_11 - 24 * SHIFT_17)
+		//+ (MULTIPLE_OF_13 - 18 * SHIFT_13)
+		//+ (MULTIPLE_OF_17 - 24 * SHIFT_17)
 	;
 	factor = 3;
 	for (;;) {